t LIBRARY OF CG.XGRKSS, 1 



^ ' # 

^UNITED STATES OF AMERICA. | 



^^ 






BOUVIER'S 
FAMILIAR ASTHONOMY 



OR, AN 



ILLUSTRATED BY CELESTIAL MAPS AND UPWARDS OF TWO HUNDRED 
FINELY EXECUTED ENGRAVINGS. 

TO WHICH IS ADDED 

A TREATISE ON THE GLOBES, 

AND A COMPREHENSIVE ASTRONOMICAL 
DICTIONARY. 



|0r % nt 0f %i\m\%, i^mXm, anJr fritot^ Stahnts. 



By HANT^AH M. BOUYIER. 



'' Lift up your eyes on high, and behold who hath created these things. 



r PHILADELPHIA: 

SOWER, BARNES & POTTS. 

37 NoKTH Third Street. 



1^5- si 



Entered according to the Act of Congress, in the year 1855, by 

CHILDS & PETERSON, 

in the Clerk's Office of the District Court of the United States for the Eastern 
District of Pennsylvania. 



STEREOTYPED BT L. JOHNSON 4 CO. C L L 1 N S, P R I N T E R. 

PHILADELPHIA. 






1 



With the ivarmest filial affection, this Volume is dedicated to 
the 3Iemo7'y of my Father, 

John BouvieVj 

to tvhose unremitted solicitude and parental instruction is due 

any merit this Work may possess. 

That it may he as unerring a Guide to the young Astronomer 

as his Works have proved to the Legal Student, is the highest 

aspiration of 

The Author. 






NOTE. 

At the solicitation of numerous Teachers^ and others interested 
in the cause of Education^ the Publishers have been induced to 
issue the present cheap edition of this work, which is adapted to 
the wants of Common Schools. 

The references to Practical Astronomy, Problems, 3Iaps, His- 
tory of Astronomy, Notes, Dictionary of Astronomy, and Tables, 
refer to the large, complete, and elegantly printed edition, which 
will be found indispensable in the 'prosecution of this delightful 
study. Published in one large octavo volume, 500 pages, sub- 
stantially bound. Price, $2.00. 

Philadelphia, 1858, 



PEEFACE. 



This work, embracing all the recent observations of the heavenly 
bodies, is intended to be a complete treatise on Astronomy, conducting 
the pupil, step by step, from the base to the summit of the structure ; 
explaining as far as practicable, by figures and diagrams, all the celestial 
phenomena, and the laws by which they are governed, without entering 
into those mathematical details which properly belong to treatises 
designed for those who propose to make Astronomy their chief study. 

This science, formerly but little taught in seminaries, now claims the 
attention of all enlightened teachers; its importance having been ac- 
knowledged by the greatest men of all ages. 

Besides elevating the mind and improving the thinking faculties, it 
is of the utmost utility to man. Without Astronomy we could have no 
proper computation of time, no true knowledge of geography, and no 
correspondence between distant nations; for, as Lacaille observes, ^^As- 
tronomy is the governor of the civil division of time, the soul of chro- 
nology and geography, and the only guide of the navigator.'' 

The present work is divided into five parts : the first treats of the 
laws which govern the heavenly bodies ; the second, of the components 
of the solar system, and the phenomena attending their movements; the 
third, of the sidereal heavens, embracing the fixed stars, clusters, and 
nebulae ; the fourth, of the principal instruments used in the observa- 
tory; and the fifth, of the use of the globes. To which is appended 
two celestial maps and a comprehensive Astronomical Dictionary, which 



b PREFACE. 

■will facilitate tlie studies of the pupil, and relieve the teacher from 
mucli explanation which would otherwise be unavoidable. The value 
of this feature of the work must be obvious to all. 

In order to carry out the method thus proposed, the form of question 
and answer has been adopted, because the plan possesses peculiar advan- 
tages. It is calculated to concentrate the attention of the pupil upon 
the subject under immediate consideration; to dwell upon every point 
until perfectly understood; to define the precise limits of each proposi- 
tion; and to afford means for enlarging the explanations without crowd- 
ing the mind with ideas but imperfectly comprehended. 

Short notes in smaller type have been introduced throughout the text, 
which serve to elucidate the figures and diagrams, and to convey more 
complete explanations of particular subjects. There is a series of notes 
at the end of the work, which will facilitate the advancement of those 
who may wish to enter more fully into this arduous yet fascinating 
study. 

The maps, figures, and diagrams have been carefully drawn, and are 
executed in the best manner. Thus it will be seen that nothing has 
been spared to render this work useful and attractive to the pupil, as 
well as to the student of riper years ; at once qualifying it to occupy a 
place in the school-room, the study, or the parlor. 

An intimate knowledge of the heavenly bodies renders them as 
familiar as friends; so that he who can be induced to turn occasionally 
from the cares and disappointments of life to the study of the heavens 
will be amply recompensed by the rational entertainment which it 
affords. 
HiLTOx, Cross-wicks, N. J., 1S56. 



CONTENTS. 



Chap. I 

II 

III 



PART I.— PHYSICAL ASTRONOMY. page 

Chap I. — General Properties of Matter 15 

Sec. 1. Gravitation 17 

Div. 1. Centre of Gravity, 19; Div. 2. Specific Gravity and 

Density 21 

Sec. 2. General Laws of Motion 21 

Sec. 3. Compound Motion 22 

Div. 1. Curvilinear Motion, 24; Div. 2. Centrifugal and 

Centripetal Forces 26 

11. — Angular Measurement 27 

PART II.— DESCRIPTIVE ASTRONOMY. 

—Solar System 29 

—The Sun 30 

— Interior Planets 34 

Sec. 1. Mercury 35 

Sec. 2. Venus 37 

IV.— The Earth 42 

V. — Exterior Planets 51 

Sec. 1. Mars 63 

Sec. 2. Asteroids 56 

Sec. 3. Jupiter 58 

Sec. 4. Saturn 61 

Sec. 5. Uranus 63 

Sec. 6. Neptune 63 

VI.— Satellites 65 

Sec. 1. The Moon 66 

Sec. 2. Jupiter's Satellites 87 

Sec. 3. Rings and Moons of Saturn 90 

Sec. A. Moons of Uranus 93 

Sec. 5. Moons of Neptune 94 

VII.— Motion of the Earth 96 

Sec. 1. Diurnal Motions of the Earth 96 

Sec. 2. Annual Motion of the Earth 100 

Sec. 3. Seasons 102 

Time 108 

■Ecliptic and Zodiac 114 

Tides 117 

Eclipses 123 

Seel. Solar Eclipses 125 

Sec. 2. Lunar Eclipses 131 

Sec. 3, Transits 134 

Div. 1. Transits of Mercury, 136 ; Div. 2. Transits of Venus 136 

Sec. 4. Occultations 138 

Comets 140 

Systems of Astronomy 155 

Sec. 1. Ptolemaic System 155 

Sec. 2. Egyptian System 156 

Sec. 3. Tychonic System 156 

Sec. 4. Copernican System 157 

PART III.— SIDEREAL ASTRONOMY. 

Chap. I.— Fixed Stars 159 

Sec. 1. Apparent Motions and Positions of the Stars 161 

Div. 1. Precession, 164; Div. 2. Nutation, 165; Div. 3. 
Refraction, 166; Div. 4. Aberration, 169; Div. 5. Proper 

Motion, 172; Div. 6. Parallax 174 

7 



VIII. 

IX, 

X. 

XL 



XIL 
XIIL 



8 CONTEXTS. 

PAGB 

Chap. II. — Distances of the Fixed Stars 177 

III.— Milky Way 180 

IV.— Magnitude of the Stars 182 

V. — Appearance of the Stars 184 

Sec. 1. Variable and Periodic Stars 184 

Sec. 2. Temporary Stars 185 

Sec. 3. Compound Stars 187 

Div. 1. Double Stars, 187 j Div. 2. Multiple Stars 189 

Sec. 4. Clusters and Nebulae 190 

Div. 1. Double Nebulae, 193 ; Div. 2. Nubeculae or Magel- 
lanic Clouds 194 

VI.— Meteors 195 

Sec. 1. Meteorites 196 

Sec. 2. Zodiacal Light 198 

VIL— Constellations 198 

Sec. 1. Zodiacal Constellations 200 

See. 2. Northern Constellations 212 

Sec. 3. Southern Constellations 240 

PART IV.— PRACTICAL ASTRONOMY. 

Chap. I. — General Properties of Light 260 

Sec. 1. Refraction of Light 261 

Sec. 2. Reflection of Light 264 

IL— The Observatory 267 

III.— Telescopes 268 

Seel. Refracting Telescopes 268 

Sec. 2. Achromatic Telescopes 269 

Sec. 3. Equatorial Telescopes 270 

Sec. 4. Transit Instruments 270 

IV.— Micrometer 274 

V. — Instrumental Adjustments 275 

*Sec. 1. The Vernier 275 

Sec.2. The Level 276 

Sec. 3. Plumb-Line 277 

Sec. 4. Artificial Horizon 277 

Sec. 5. Collimator 278 

^ec. 6. Transit Clock 279 

VI.— Graduated Circles 281 

Sec. 1. Mural Circle 281 

Sec. 2. Transit Circle 284 

Sec. 3. Altitude and Azimuth Instrument 287 

PART v.— TREATISE ON THE GLOBES. 

Chap. I. — Problems on the Terrestrial Globe 28? 

IL — Problems on the Celestial Globe 307 

PART VI.— HISTORY OF ASTRONOMY. 

Chap. I. — Sec. 1. From the Earliest Times to the Christian Era 320 

Sec. 2. Astronomical Instruments in use from the Earliest Times to the 

Christian Era 327 

II.— Sec. 1. From the Christian Era to the year 1600 329 

Sec. 2. Astronomical Instruments invented from the Christian Era to 

the year 1600 335 

III. — Sec. 1. From the beginning of the Seventeenth to the end of the 

Eighteenth Century 336 

Sec. 2. Astronomical Instruments in use from the beginning of the 

Seventeenth to the end of the Eighteenth Century 341 

rV. — History of Astronomy from the beginning to the middle of the Nine- 
teenth Century 343 

Notes 345 

Astronomical Dictionary 411 



\ 



^nlxn)iutim. 



Q. What is Astronomy ? 

A, The science which treats of the heavenly bodies^ describes 
their appearances^ and the laws by which their motions are go- 
verned. 

Q. Into how many parts may the science of Astronomy be divided ? 

A, Into four parts. Physical^ Descriptive^ Sidereal, and Prac- 
tical Astronomy. 

Q. What is Physical Astronomy ? 

A. Physical Astronomy treats of the motions of the heavenly 
bodies, and the laws which operate to produce them. 

Q. What is Descriptive Astronomy ? 

A. Descriptive Astronomy is a relation or description of the 
appearances of the heavenly bodies belonging to the solar system. 

Q. What is Sidereal Astronomy ? 

A. Sidereal Astronomy treats of the Fixed Stars, Nehulce, &e. ; 
or those bodies which do not belong to the solar system. 

Q. What is Practical Astronomy ? 

A. Practical Astronomy treats of astronomical instruments, 
and their application. 

The following definitions are inserted for the benefit of those who have never studied 
spherical Geometry ; a knowledge of the circles of the sphere being of the utmost im- 
portance to the right understanding of the principles of Astronomy. 

A circle is a figure contained by a uniform curved line, called 
its circumference, which is everywhere equidistant from a point 
within, called its centre. 

Fig. 1. 

An Arc of a circle is at>n j part of the circum- 
ference ; consequently, in fig. 1, the dotted line, 
and also the black one, are arcs of the circle, be- 
cause neither of them constitutes the wJiole cir- 
cumference, but only a part. 



10 



BOUVIER S FAMILIAR ASTROXOMT. 



Pig. 2. 




Fig. 3. 




The Diaineter of a circle is a line drawn through 
B the centre and terminating at the circumference. 
A B is the diameter of the circle. Fig. 2. 



The Radius of a circle is a line drawn from 
D the centre to the circumference. C D is the ra- 
dius of the cii'cle in /^. 3. 



A Semicircle is half the circle, or a segment 
cut off by a diameter. A C B is a semicircle. A 
half circumference is sometimes called a semi- 
circle. It contains 180°. 



D 



A Quadrant is half a semicircle, or one-fourth part of a whole 
circle. A quarter of a circumference is sometimes called a quad- 
rant. B E C and C E A [jig. 4) are quadrants, all of which con- 
tain 90°. 



A G-reat circle of a sphere is one whose plane 
B passes through the centre of the sphere, as A B, 




{fig- 5.) 



A Lesser or Small circle of the sphere is that whose plane does 
not pass through the centre of the sphere, but divides it into two 
unequal parts ; as e e, in fig. 5. 

The Axis of the earth, or of a heavenly body, is that diameter 
about which it performs its diurnal revolution. In fig. 5, P C P 
represents the axis of the earth. 

The Poles of a sphere are the extremities of its axis. P P 
{fig. 5) represent the poles of a sphere. 

The Equator is an imaginary circle on the surface of the earth 
equidistant from the poles. In fig. 5, A B represents the equator. 



INTRODUCTION. 



11 





The Equator of the heavens^ or Equinoctial, is the terrestrial 
equator extended to the surface of the starry sphere. 

Fig. 6. 

The Tropics are two lesser circles of the 
sphere, situated at the distance of 23° 28' north 
and south from the equator. The tropic (c c, 
fig. 6) north of the equator is the tropic of Can- 
cer ; the tropic (N N) south of the equator is 
the tropic of Capricorn. 

The Polar circles are two lesser circles situated at the distance 
of 23° 28' from each pole. The north polar, or Arctic circle, is 
marked a a, {fig. 6,) and the south polar, or Antarctic circle, is 
marked S S, 

Fig. 7. 



Concentric circles are circles of different dia- 
meters drawn round a common centre. Fig. 7. 



A Sphere is a solid figure, every part of whose surface is 
equally distant from a point within, called its centre. 

Fig. 8. 

A Spheroid is a figure nearly spherical. It may 
be prolate or oblate. A lemon is in the form of a 
prolate spheroid, and an orange is in the form of 
an oblate spheroid. The earth is an oblate spe- 
roid, though much less compressed than the figure. 

Zenith, that point in the heavens directly overhead. 

Nadir is that point in the heavens directly under the feet of 
the observer. The zenith and nadir are the poles of the horizon. 

Vertical circles of the sphere are great circles passing through 
the zenith and nadir. 

The prime vertical is a circle perpendicular to the meridian, 
which passes through the east and west points of the horizon. 

A Hemisphere is the half of a globe or sphere. 

Meridians are imaginary circles drawn through the poles, and 
cutting the equator at right angles. 

The celestial sphere is the concave surface of the heavens sur- 
rounding the earth in all directions. 




12 



BOUVIER S FAMILIAR ASTROXOMT. 



Axis of the heavens is the axis of the earth extended to the 
concave surface of the celestial sphere. 

Fig. 9. 



Fig. 10. 



An Angle is the opening of two straight lines 
having different directions and meeting in a point. 



A line is perpendicular to another when it in- 
clines no more on one side than on the other, 
making the angles on both sides of it equal. Fig. 10. 



Fig. 11. 




An Acute angle is less than a 
right angle ; that is, less than 90°. 
ABC [fig. 11) is an acute angle. 



A Right angle is that which is made by one line perpendicular 
to another; or, when the angles on each side of a perpendicular 
are equal to one another. In jig. 11, D B is perpendicular to 
F C, and the angles C B D and DBF are right angles, and equal 
to each other. 

An Obtuse angle is greater than a right angle ; that is, it con- 
tains more than 90°. C B E and A B F are obtuse angles. 

Fig. 12. 



A Triangle is a figure bounded by three sides. 



A Parallelogram is a figure which has its 
opposite sides equal and parallel. 




Fio-. 13. 



Fig. 14. 



Parallel lines are always at the same per- 
pendicular distance from each other; they 
never meet, though indefinitely produced. 



INTRODUCTION. 13 

Fig. 15. 




An Ellipse is an oblong plane 
figure, having two points, called its 
foci, around which the figure is de- 
scribed. 



The transverse axis of an ellipse is the longer one A B,fig, 15. 

The conjugate axis is the shorter one, marked C D. 

Latitude on the earth is the distance of a place north or south 
of the equator, measured on the meridian. 

Latitude of a heavenly body is its distance north or south of 
the ecliptic. 

Longitude of a place on the earth is its distance east or west 
from any given meridian. 

Longitude of a heavenly body is its distance from the first 
point of Aries, reckoned eastward on the ecliptic. 

Declination of a heavenly body is its distance north or south 
of the equinoctial, measured on the meridian. 

Right ascension. — The distance of any celestial body eastward 
from the point Aries, reckoned on the equinoctial. 

The Ecliptic is that great circle in which the sun appears to 
move, but which is the real path of the earth. The poles of the 
equator are the poles of the earth ; but the poles of the ecliptic 
do not correspond with those of the equator, but are 23° 28' dis- 
tant from it ; therefore the ecliptic must be inclined 23° 28' to 
the equator. If two rings be placed one within the other so that 
their circumferences shall not coincide, they will be inclined to 
each other, unless they be so placed as to cross each other at right 
angles, in which case they will be perpendicular to each other. 

The Equinoxes, or Equinoctial points, are the points where 
the ecliptic and equator intersect each other. The vernal equinox 
is that point of the ecliptic in which the sun appears about the 
21st of March ; the autumnal equinox is his apparent place about 
the 22d of September. At both these points the sun is vertical 
to all inhabitants of the earth living at the equator. 

The Solstices, or Solstitial points, are those points of the ecliptic 
most distant from the equator. The solstices are situated 90° 
from the equinoxes. The summer solstice occurs about the 21st 
of June, and the winter solstice about the 22d of December. 



14 



BOUVIER S FAMILIAR ASTRONOMY. 




arc A C. For 

for it is drawn 



Fig. 16. A. Sine of any arc is the straight line drawn 

from one extremity of the arc perpendicular to 
the radius drawn to the other extremity. Thus 
A B is the sine of the arc A C, because it is 
drawn from A, which is at one extremity of the 
arc A C, and is perpendicular to the radius, 
which is half the diameter C D. Therefore, 
the radius meets the other extremity C of the 
the same reason A B is the sine of the arc A D, 
from A, which is one extremity of the arc A D, 
and is perpendicular to the radius, which, being one-half the 
diameter C D, meets the other extremity D of the arc A D. 
Therefore every sine is equal to two arcs, which together form a 
semicircle. 

Fig. 17. 



A Tangent is a straight line which touches 
but does not cut a curve. A B is a tangent 
to the circle C D. Fig. 17. 



B A Chord is a straight line connecting the two 
extremities of an arc. A B is a chord common 
to the two arcs A D B and A E B, which to- 
gether constitute the whole circumference. -F^^. 18. 




PART I. 



"Enrich me -with, the knowledge of Thy -works ! 
Snatch me to Heaven; Thy rolling -wonders there, 
World "beyond -world in infinite extent. 
Profusely scattered o'er the blue immense, 
Sho-w me. Their Motions, Periods, and their Laws, 
Give me to scan. Through the disclosing deep 
Light my blind -way." 

Q. Are the laws -which govern the heavenly bodies similar to those observed 
on the earth ? 

A. They are ; many of the same laws which are observed on 
the earth may be traced to the most distant parts of the universe. 



CHAPTER I. 

§mtxnl ^§upxtm 0f Patter. 



Q. What is matter? 

A. Matter is any thing which is the object of our senses. 

Q. Is every thing -which -we can see, taste, or feel, composed of matter ? 

A. Yes ; also whatever is the object of the senses of smelling 

or hearing. 

Q. Ho-w can the objects of the senses of smelling or hearing be composed of 
matter ? 

A. The odoriferous ^particles which emanate from bodies are 

material, as also the air, the concussion of which produces sound. 

Q. What are the general properties of matter ? 

A. The principal qualities of matter are magnitude, im-pene- 
trabilitg, divisibility, compressibility, dilatability, and inertia. 
Q. What is magnitude ? 
A. It means size or bulk. 
Q. Have all bodies magnitude ? 

A. All bodies occupy space, therefore they must have magni- 
tude. 

15 



16 bouvier's familiar astronomy. 

Q. TVliat is meant by the impexetkabilitt of matter ? 
A. It cannot be penetrated without disturbing its component 
parts. 

Q. What is the result if a gimlet penetrate a board, or an oar penetrate the 
Trater ? 

A, In the first instance the component parts of the hoard are 
displaced by the gimlet; and in the second, the particles of water 
are displaced by the oar. 

Q. What is understood by the DmsiBniTT of matter ? 

A. Its capability of being separated or divided. 

Q. Are all bodies divisible ? 

A. All matter is divisible, however minute it mav be. [See 
Xote 1.) 

" Animalcules have been discovered so minute that thousands of them do not equal a 

grain of sand in bulk, yet, as these creatures have life and the power of motion, they 
may be supposed to be possessed of the functions of living beings. How exceedingly 
minute must be their heart, lungs, nerves, <£c. 1 Yet the particles of light are infinitely 
more minute than these." 

According to Ehrenberg, a cubic inch of water may contain more than 800,000,000,000 
of animalcule: and a single drop placed under the microscope wUl be seen to hold 
500,000,000; an amount, perhaps, nearly equal to the whole number of human beings 
on the globe I 

"In -"nc-'asaiii species c: the insect kini, 
Lest to the naked eye, so -w-ondrous small. 
Were jnillions joined, one grain of sand -would c over all ; 
Yet eacli -witliin its little bulk contains 
A heart, -which drives the torrent thxcngh its Teins ; 
IJ'j-scles to move its limbs ari^t; a brain. 
-Lni nerves disposed for pleasure, and for pain; 
Zyes to distinguish, sense. Tvherecy to knew 
What's good, or bad, is, or is not, its foe." 

Q. What is meant by the compressibility of matter ? 

A. The diminishing its hulk or size^ without destroying its 
component parts. 

Q. Suppose a goblet to be inverted over a basin of water just so as to permit 
its rim to touch the surface of the water, what will the goblet contain ? 

A. The goblet will be filled with air. 

Q. What would be the effect if the goblet should be forced down into the 
water, without permitting the air to escape ? 

A. The water would rise some distance inside of the goblet, 
thereby proving the compressibility/ of the air. 

Q. How does this prove the compressibility of the air ? 

A. The air in the goblet was compressed by forcing the goblet 
down into the water without permitting the air to escape, causing 
it to occupy less space than before. 

Q. What other property of air does this experiment prove ? 

A. It proves the impenetralility of air; for it prevented the 
water from penetrating into its mass. 



GENERAL PROPERTIES OF MATTER. 17 

Q. Some bodies after being compressed return to their former dimensicns; 
what is this property called ? 

A. It is known by the name of elasticity, 

Q. Are all elastic bodies compressible ? 

A. Yes; but all compressible bodies are not elastic, that is, 
they will not return to their original forms after being compressed. 

India rubber is an elastic body. 
Q. What is meant by the dilatability of matter ? 
A, Its capability of extension without increasing its mass. 

Take a bladder, the mouth of which is tied, so as to prevent the escape of the air from 
the inside, and place it under the receiver of an air-pump. Exhaust the air in the 
receiver, and the bladder will be observed to swell. This is owing to the external pres- 
sure of the surrounding air having been removed, which permits that in the bladder to 
dilate or expand. 

Q. What is meant by inertia ? 

A. That quality of matter which implies indifference to either 

rest or motion — passiveness. 

Q. What would be the effect if a person be riding on horseback at a rapid 
pace, and the horse stop suddenly ? 

A. He would be thrown over the horse s head. 

Q. Why would he be thrown over the horse's head ? 

A. Because by reason of his inertia he would continue in the 
motion first imparted to him by the horse. 



SECTION I. 

^rabitEliotr. 

"That very law -which moulds a tear, 
And bids it trickle fxora its source, 
That law preserves the earth a sphere. 
And guides the planets in their course." 

Q. What is meant by gravitation ? 

A. That power which impels bodies towards each other. 

Q. Why does a stone, when dropped from the hand, fall to the earth? 

A, Because the powerful attraction of the earth's greater mass 
impels the smaller body to move towards it. 

Q. What would be the effect if the earth and the stone were of equal mass ? 

A, They would fall towards each other, and meet half way. 

Q. If a stone were abandoned to itself at a great distance from the earth's sur- 
face, would it fall towards the earth ? 

A. Yes ; if the stone were several thousand miles distant it 

would as certainly fall to the earth as it does when dropped at the 

distance of a few feet. 

Q. Would a stone, a thousand miles from the earth, weigh as much as it would 
at the eai'th's surface? 

A. No ; the power of gravitation decreases above the earth's 
surface, in proportion as the square of the distance from the 
earth's centre increases. 



18 bouvier's familiar astronomy. 

Q. Explain this. 

A. A body which, at the surface of the earth weighs one pounds 
would at 4000 miles above the surface, that is, at double the dis- 
tance of the surface from the centre, only weigh a quarter of a 
pound. 

As the moon is about 240,000 miles from the earth, the attraction of the earth at that 
distance must be equal to the square of 240,000 divided by the square of 4000, the dis- 
tance of the centre from the surface of the earth. By this method it is found that a 
body weighing one pound at the surface of the earth, would weigh the 5^ g of a pound 
at the distance of the moon. 

Q. With what velocity do bodies fall, which are situated near the surface of 
the earth ? 

A. Bodies near the earth's surface fall sixteen feet in the first 
second, three times sixteen in the next second, five times sixteen 
in the third second, and so on, continually increasing, according 
to the odd numbers 1, 3, 5, 7, 9, &c. 

Q. What is the law of falling bodies ? 

A. The space through which a hody falls increases in propor- 
tion to the square of the time it occupies in falling. 

Q. Illustrate this law. 

A, If a stone require four seconds to reach the ground from a 
high tower, the tower will be two hundred and fifty-six feet high. 
Because the stone would fall 16 feet in the first second, 3 times 
16 in the next, 5 times 16 in the third, and 7 times 16 in the 
fourth, which, added together, make the sum of 256 feet. 

Q. Is there a shorter method of obtaining the same result ? 

A. Yes ; by squaring the number of seconds, and multiplying 
the result by 16 ; thus 4 X 4 X 16 = 256. 

Q, Does the attraction of gravitation act on all bodies ? 

A. It does ; without any regard to their figure or size. 

Q. Does it act more powerfully on some bodies than on others ? 

A. It acts in proportion to the quantity of matter which they 
contain. 

Q. Would a greater force of gravity be exerted on a body weighing ten 
pounds, than on one weighing one pound ? 

A. A ten times greater force would be exerted on the body 
weighing ten pounds. 

Q. Would two bodies of unequal weight faU to the earth in the same space of 
time? 

A. Yes; they would fall to the earth with equal velocities. 

(See Note 2.) 

Q. If, by the attraction of gravitation, all bodies are drawn to each other, why 
do they tend to the earth ? 

A. The earth being so much more vast than any body on or 
near its surface, the effect of this attraction between smaller bo- 
dies is not perceptible. {See Note 3.) 



GENERAL PROPERTIES OF MATTER. 



19 



DIVISION I. — CENTRE OF GRAVITY. 

Q. What is the centre of gravity ? 

A, It is that point of a bodj upon which, if it be freely sus- 
pended, it will rest. The centre of gravity is no other than the 
centre of parallel and equal forces. 

Q. Have all bodies a centre of gravity ? 

A. Yes; there is a point in every body, of whatever form 
it may be, in which the forces of gravity may be considered as 
U7iited. 

Q. In what direction does a body, when not supported, endeavour to fall? 

A. In a line drawn from its centre of gravity towards the cen- 
tre of the earth, 

Q. What is that line called along which every body when unsupported en- 
deavours to fall ? 

A. It is called the line of direction. 

Q. Why does a body sometimes stand and at other times fall ? 

A. If the line of direction fall within the base, it will stand ; 
but if it fall without the base, it will fall. 



Fig. 19. 




There is a tower in Pisa, in Italy, which leans so much as to appear as if it would 
fall; but its line of direction falls within the base, so that it will be likely to stand in 
its present position until its materials fall into decay. 

Q. In what part of a globe is the centre of gravity placed ? 

A. In the centre. 

Q. The earth is an oblate spheroid ; in what part of the earth is its centre of 
gravity ? 

A. In its centre. 

Q. In irregularly formed bodies where is the centre of gravity situated ? 
A. It is always situated near the largest portion of the mass. 



20 



BOUVIER S FAMILIAR ASTRONOMY. 




Fig. 20. In order to find the centre of gravity of 

the triangle ABC, {fig, 20.) draw a line 
from the point A to D, situated at the mid- 
dle of the base. This line A D equally 
divides all those lines which may be drawn 
parallel to B C : the centre of gravity will 
be found on the line A D. From the point 
C to the middle of the opposite side A B, 
draw the line C E : the centre of gravity 
will be found on this line also ; and as it lies 
somewhere on the line A D, it must be at 
their intersection at F. Now draw the line 
ED; and as A : D E : : A F : F D ; but 
A C is double D E ; then A F must also be 
double F D, and consequently the centre of gravity of a triangle is situated on a line 
drawn from the highest point, and at two-thirds of its length from its base. 
Q. In what part of a cone is the centre of gravity situated ? 

^. At a point about one-fourth of its height from its base. 

Q. Why "will a cone stand more fikmlt than a cylinder ? 

A. Because in the cone the centre of gravity is situated near the 
base, while that of the cylinder is midway between its extremities. 

Q. If persons were to be overtaken by a squall of wind while in a small boat, 
in -what position should they place themselves ? 

A. They would incur less risk by sitting in the bottom of the boat. 

Q. Why would that be the safest position ? 

A. Because by sitting in the bottom of the boat they lower 
their centre of gravity. 

Q. Why does a man stand with his feet apart when driving a cart ? 

A. Because by increasing his base he lowers his centre of gravity. 

The broader the base, the nearer is the line of direction to the middle of it. 
Q. Why are all spherical bodies more easily moved than bodies of any other 
form? 

A. Because the base is a mere point, and consequently the line 
of direction is easily moved beyond the base. 

Q. Suppose two balls of equal size be united by a stiff wire, where would the 
centre of gravity be situated ? 

A. At a point in the wire equidistant from each of them. 

Q. Suppose one of the balls to be three times heavier than the other, where 
would the centre of gravity be situated ? 

A. It would be three times nearer to the large ball than to the 
small one. {See Note 4.) 

Fig. 21. 




9 



The sun is so much larger than the earth and all the planets put together, that the 
centre of gravity of the solar system is situated within the globe of the sun. 



GENERAL PROPERTIES OF MATTER. 21- 

DIVISION TI. SPECIFIC GRAVITY AND DENSITY. 

Q. What is understood by specific gravity ? 

A. It denotes the weight of a body, as compared with the 
weight of an equal hulk of some other body taken as a standard. 

Q. What OTHER BODY IS commonly used as a standard of unity ! 

A. The standard commonly used is distilled water, at the tem- 
perature of 60° Fahrenheit ; but any other substance would an- 
swer as well as water. 

Q. What is meant by density ? 

A. By the density of a body is meant its mass, or quantity of 
matter, compared with that of an equal volume of water, or some 
other body taken as a standard. 

The density of a body is the relation of its weight to its volume. 
Q. Would a cubic inch of water and a cubic inch of steel hold each other in 
equilibrium ? 

A. No ; although their masses measure the same, the steel is 

nearly eight times more dense than the water. 

Q. How MAjJY cubic inches of water would be required to balance the cubic 
inch of steel ? 

A. It would require nearly eight cubic inches of water to ba- 
lance one of steel. 

The specific gravity of steel is nearly eight times that of water. Thus, we say ice is 
less dense than water, and water less dense than gold, silver, steel, <fcc. 



SECTION II. 

(!^eneral ITafos of potion. 

Q. What is motion ? 

A. Motion means change of position; the act of moving. 

Q. When is a body said to be in motion ? 

A. When it successively occupies different positions at differ- 
ent times. 

Q. How is motion classified, or divided ? 

A. It is classified into three kinds, viz. Absolute motion. Rela- 
tive motion, and Apparent motion. 
Q. What is absolute motion ? 

A. Absolute motion is a change of position with regard to a 
stationary point. 

Q. What is relative motion ? 

A. Relative motion is a change of position in a body, with 
respect to another body also in motion. 

Q. What is apparent motion ? 

A. Apparent motion is due to a change of place in the ob- 
server, which produces a change in the lines of vision between 
certain fixed objects and himself. 



22 BOUVIER'S familiar ASTRONOxMY. 

Q. When is a body said to be in a state of rest ? 

A. When a body is immovable or motionless, it is said to be in 
a state of rest. 

Q. Can a body be said to be in a state of absolute best ? 

A. No ; as it is supposed there is nothing in nature without 
motion, there can be no absolute, only relative rest. 

Q. Has a body in a state of rest the power to change its position ? 

A. A body in a state of rest cannot impart motion to itself. 

Q. Has a body in motion the power to change its bate of motion ? 

A. A body in motion cannot of itself cha>nge or annihilate its 
condition of motion. 

Q. How can a change of motion be produced ? 

A. It requires some external resistance to put an end to mo- 
tion, and also some force must be impressed on a body in order to 
put it in motion. 

Q. What resistance does a ball receive when fired from a camion ? 

A. Its motion is checked 

1. By the attraction of gravitation, and 

2. By the resistance of the air. 

Q. If a marble be rolled over smooth ice, or a gravel walk, what finally stops 
its motion ? 

A. It is checked hj friction. 

Q. What is the impetus or quantity of motion of a moving body called ? 

A. It is called momentum, or force of motion. 

Q. On what does the momentum of a moving body depend ? 

A. It depends 

1. On its weight, and 

2. On the velocity of its motion. 

Q. How does a moving body acquire its momentum ? 

A. It acquires it from some other agent already in motion. 

Q. Can one body confer momentum on another without losing the same amount 
itself? 

A. No ; momentum is always a communicated property . Every 
body loses the same amount of momentum which it imparts ; and 
every body that acquires momentum takes what another has lost. 
{See Note 5.) 

SECTION III. 

Compotmb Potion. 

Q. What is compound motion ? 

A. Compound motion is that which is produced by the combi- 
nation of two or more forces, acting in different, but not opposite 
directions. 

Q. How does a body move when impelled in two different directions ? 

A. It moves in a direction intermediate to those two forces. 



GENERAL PROPERTIES OF MATTER. 



23 



Fig. 22. 



Suppose R R to be a river, and that a boat starts from A to 
cross the stream, and endeavours to arrive at the point C. If, 
at the same time, the current is running towards D, the boat 
goes neither to C nor D, but moves on to B in a direction inter- 
mediate to the current and the force of the rower. Each force 
modifies the influence of the other, and brings its power to bear 
in a new direction, without destroying it, unless the two forces 
act in directly opposite lines, and with equal influence; in 
which case the one entirely neutralizes the other. 



Q. What KIND of motion is that called which is the result of two or more com 
bined forces ? 

A. It is called compound motion, 

Q. What is the line called which indicates the direction of this compound 
motion ? 

A, It is termed the mean or resultant. 

In the foregoing figure the line A B is the resultant of the two forces A D and A C. 
It will be seen that the lines A D and A C, which represent the forces, form two sides 
of a parallelogram, the diagonal of which, A B, is the mean ; hence it is called the 
parallelogram of forces. 




Fig. 23. 



The resultant of two forces is found thus. In Jig. 23 
we have the two forces A C and A B acting in different 
directions ; the one of itself would have driven a body 
from A to C, and the other from A to B. But it obeys 
both farces by following the line A D, which is the mean 
or resultant. If the lines C D and B D be drawn parallel 
to A C, their point of intersection at D forms the extre- 
mity of the resultant A D. 



Q. Do the different forces act with the same power at any angle ? 

A, No ; the combined forces act with more power at an acute 
than at an obtuse angle. As the angle decreases, the effect of the 
united forces increases. 

Fig. 24. 
A C 





In Jig. 24 the point A, from which the combined forces A C and A B proceed, is an 
acute angle, and hence the diagonal which represents the resultant will be increased. 



24 



BOUVIER S FAMILIAR ASTRONOMY. 



Now, should the angle C A B be reduced till it vanishes, or, which is the same thing, 
when the sides A C and A B coincide, the combined forces will have their greatest eflFect. 
But should the forces be equal, and act in opposite directions in a straight line, they 
will destroy each other. 

DIVISION I. — CURVILINEAR MOTION. 

Q. What is curvilixear motion? 

A. Curvilinear motion is motion in a curved line, 

Q. Suppose a blow be given to a ball suspended by a thread, in what direction 
would it move ? 

A. Its motion would be in a curved line, because the thread 
would prevent it from moving in a straight one. 

Fig. 25. 



/"' 



\ 



\ 



M- 



If a blow be given to the ball at A, it would move in the straight line A T, if it were 
not attached to the thread which is fastened at the point C ; consequently it is held by 
the thread, and moves in the curved line A B D E. 

Q. What is a projectile ? 

A. A body which is projected or thrown, 

Q. Does a projectile always describe a curved line ? 

A, Yes ; if it be thrown in any other than a vertical direction, 
it will describe a curve. 



Fig. 26. 
bed 



f 



9 X^ 

\ 



The curve aghik represents the path of a projectile. 
By reason of the first impulse, it would require one 
second to travel over the space a h, and if there was no 
such force existing as gravity, it would traverse the 
space 6 c in the next second, and so on. But in the first 
second it fell to g, in the next to h, and in the third, in- 
stead of being at d, it has moved to i, and in the fourth 
second it has arrived at k. 



GENERAL PROPERTIES OF MATTER. 



25 



Q. What is the course described by a projectile when thrown in any other 
than a vertical direction ? 

A. The curved line described by a projectile in free space is a 
parabola. 

Q. Why does not a projectile describe a parabola when near the surface of the 

EARTH ? 

A. Because the air offers sufficient resistance to change its curve. 

Fig. 27. 

A 




Taking into consideration the resistance of the air, it will be seen that the curve 
described is not a perfect parabola. The line A B is its asymptote. 

Asymptote. — A right line, which continually approaches nearer and nearer to a curve 
without ever meeting it. 

Q. Could a body be made to move in a curved line by the action of a single 
FORCE ? 

A. No ; every body which moves in a curved line must be acted 
on by at least two forces. 



Fig. 28. 



Let C {fig. 28) be the point towards which a 
body at A is continually attracted ; but let us sup- 
pose that the body receives at the same time an 
impulse in the direction A D. In the first second 
the central force tends to drive the body from A 
to B, and the lateral force impels it from A to D j 
it follows the diagonal A E of the parallelogram. 
As soon as it arrives at E, it is still inclined to 
proceed in the straight line to G, but is attracted 
by the central force C towards F ; consequently 
it pursues the path E H, the diagonal of the paral- 
lelogram E F H G. When at H, owing to the 
same reason, it pursues the path H L. Thus the 
body describes the curve A E H L. But as the 
central force acts equally and without intermis- 
sion, the path the body would pursue would be a 
perfect curve. The diagram, however, serves to 
elucidate the principle. 



Curvilinear motion is no other than a series of uninterrupted movements in straight 
lines, forming very obtuse angles. 




26 BOUVIER's familiar ASTRONOxMY. 



DIVISION II. CENTRIFUGAL AND CENTRIPETAL FORCES. 

Q. How is CURVILINEAR MOTION, OP motion round a centre, produced ? 

A. It is produced by two powers, one of which is called the 
centrifugal^ the other the centripetal force, 

Q. If a stone be whirled in a sling, what two forces act upon the stone ? 

A. The force or power of the string^ which retains the stone, 
and the force or power of the hand, which keeps it in a circular 
motion. 

Q. If the string were cut while being whirled round, would the stone fly ofi^ in 
a CURVED line ? 

A, No; the stone would fly off in a line at right angles with 
the string. 

Q. What causes the stone to fly ofi" at right angles with the string ? 

A. T\iQ projectile force given to it by the hand impels it in a 
straight line ; but the force of the string changes its course into 
that of a curve. 

Projectile Force. — That power which impels a body onward. 
Q. What is this projectile force called when applied to the planets ? 
A. It is. called centrifugal force. 

Centrifugal. — Flying or receding from the centre. 

Q. What would be the effect if some dry sand were placed on the edge of a 
wheel when the wheel is revolving rapidly ? 

A. The particles of sand would fly off the rim at a tangent; 
that is, at right angles to the spokes of the wheel. 

Q. What force causes the particles of sand to fly oflf the edge of the wheel ? 

A. It is the projectile or centrifugal force. 

Q. Do the particles of sand fly off the wheel in a curved, or a straight line? 

A. They fly off in straight lines, at right angles to the spokes ; 

but the force of gravity brings them to the ground in a curve. 

Q. What is that force or power which is constantly urging a revolving body 
towards the centre of motion ? 

A. It is called centripetal force. 

Centripetal, — Tending to the centre. Centripetal force and the attraction of gravita- 
tion are terms of the same import. 

Q. How may centrifugal force be increased ? 

A. By increasing the velocity of the revolution oi a body. Cen- 
trifugal forces are proportioned to the squares of their velocities. 

Q. Give an example to illustrate this rule. 

A. If the velocity of a revolution be increased three times, the 
centrifugal force will be nine times greater, because nine is the 
square of three ; if it were increased twenty times, the centrifugal 
force, for the same reason, would he four hundred times greater. 
{See Note 6.) 



ANGULAR MEASUREMENT. 



27 



CHAPTER II. 

Q. What is an axgle ? 

A. It is the opening between two straight lines, drawn from 
the same point ; or, in other words, the amount of divergence 
separating two straight lines drawn from the same point. 

Fig. 29. 



In the accompanying figure, the opening be- 
tween the two straight lines a m and c m con- 
d stitutes an angle. The openings between each 
of these lines form four diflFerent angles, the'point 
m being the vortex, or angular point. 



Q. How can the amount op divergence between two straight lines, drawn 
from the same point, be estimated ? 

A. In order to determine the size of an angle, the circle is em- 
ployed. For this purpose observe what proportional part of the 
circumference of the circle is contained between the lines. 

Fig. 30. 



Draw a circle round C, 
[fig. 30,) and it will be seen 
that one-fourth of the cir- 
cular space is included be- 
tween the lines C A and C 
B. The amount of the an- 
gle or space included be- 
tween the lines C A and C B, 
is measured off on the cir- 
cumference of the circle. 




28 bouvier's familiar astronomy. 

Q. What kind of an angle is that which includes the fourth part of a circle 
between its sides? 

A. It is called a right angle. 

A C B [fig, 30) comprises the fourth part of the circle, therefore it is a right angle. 
Observe a certain spoke of a wheel when it points directly upwards, and turn the wheel 
until that spoke is horizontal, that is, parallel with the horizon, and you will have 
caused the spoke to have changed its position by the fourth part of a circle, or a right 
angle. 

Q. How many right angles are there contained -within the circumference of 
a circle ? 

A. Every circle comprises four right angles within itself. 
Therefore, every right angle must equal the fourth part of a circle. 

Q. Into how MANY parts is a circle usually divided, for the purpose of mea- 
surement by angles? 

A. The circle is divided into 360 equal parts called degrees, 
and each degree into 60 equal parts called minutes, and each 
minute into 60 equal parts called seconds. 

60 seconds marked " = one minute. 
60 minutes " ' = one degree. 
360 degrees " ° = one great circle. 

Seventy-five degrees, forty minutes, and fifty-three seconds, is written thus : 75° 40' 53". 
Q. How many degrees are contained in a right angle ? 

A. As a right angle is the fourth part of a circle, and as a 
circle contains 360 degrees, it follows that a right angle must 
contain the fourth part of 360 degrees, which is 90 degrees. 

Q. For WHAT PURPOSE is this subdivision of circles into angular segments? 

A. It is for the purpose of enabling observers to determine the 
apparent position of objects with great accuracy. 

Q. What is meant by the apparent position of objects? 

A. Their position, so far as it can be determined by sight alone. 

The apparent position must not be confounded with the absolute or real position cf 
objects. Objects may be very little asunder, apparently, which are in reality very far 
apart. Thus, two trees in a line with the eye may appear to touch, although they may 
be separated by several yards. 

Q. Why is angular measurement of so much importance in Astronomy ? 

A, Because by angular measurement the astronomer discovers 
the distances and dimensions of the heavenly bodies. (See Note 7.) 



PART II. 



CHAPTER I. 

$ahx Bptm. 

, 2'h.e Sun revolving on his axis turns, 
And -witli created fire intensely TDirrns ; 
Impell'd the forcive air, our earth supreme 
Rolls -with the planets round the solar gleam- 
first. Mercury completes his transient year. 
Glowing refulgent -with reflected glare ; 
Bright Vbnxts occupies a wider way. 
The early harhinger of night and day. 
More distant still, our Globe terraqueous turns. 
Nor chills intense, nor fiercely heated "burns ; 
Around her rolls the lunar orb of light. 
Trailing her silver glories through the night. 
Beyond oxlt glohe, the sanguine Mars displays 
A strong reflection of primeval rays : 
The group of Asteroids in order move 
Between the planets Mars and mighty Jove. 
Next, belted Jupiter far distant gleams. 
Scarcely enlightened with the solar beams. 
"With FOUR unfixed receptacles of light. 
He towers majestic through the spacious height ; 
But farther yet the tardy Saturn lags. 
And eight attendant luminaries drags ; 
Investing with a double ring his pace. 
He travels through immensity of space. 
Next, see Uranus wheeling wide his round 
Of fourscore years ; not unassisted found 
By human eye ; the telescope displays 
Him, with ses moons, to philosophic gaze. 
Still more remote, pale Neptune wends his way; 
Le Verrier's sMll divined his distant I'ay. 
His lengthened year, by his slow-moving pace. 
A hundred sixty-four of ours may trace." 

Q. Why is the solar system so called ? 

A. It is called solar from Sol, the Sim. 

Q. Of what does the solar system consist ? 

A. It consists of the Sun^ the planets^ with their sat ell it a^ or 
moons, and the comets, 

29 



30 bouvier's familiar astronomy. 

Q. Where is the Sun situated ? 

A. In the centre of the system, the planets and comets re- 
volving around him. 
Q. What are the planets ? 

A. Those bodies with well-defined discs, which revolve round 
the Sun, and receive their light and heat from him. 

The planets and satellites always show well-defined discs when viewed through the 
telescope, although to the naked eye some are entirely invisible. 
Q. What are satellites ? 

A. The moons of a planet. Our Moon is a satellite of the 
Earth. 

Q. AVhat are comets ? 

A. They are bodies which revolve around the Sun, but which, 
unlike the planets, have generally no well-defined discs. 



CHAPTER O. 

. ®|e Sun, 

" Of all the products of the fertile earth, there is not one which is not called into 
activity, and brought to maturity, by the action of the Sun ,• neither is there a streak 
upon a leaf, or a tint upon a flower, but what is limned by the orb of day. Thus, the 
heavens become, in the strictest sense of the word, the keys to the knowledge of univer- 
sal nature." 

"Fairest of beings ! first created light; 
Prixue cause of beauty! for from thee alone 
The sparkling gem, the vegetable race, 
The nobler worlds that live and breath their charms. 
The lovely hues peculiar to each tribe, 
From thy unfailing source of splendour dra-w ! 
In thy pure shine with transport I survey 
This firmament, and these her rolling -worlds. 
Their magnitudes and motions." 
Q. What is the Sun ? 

A. An enormous globe of dense matte?', from which light and 
heat are constantly emanating. 

Q. How can it be proved that the Sun is composed of dense matter ? 
A. It is known to exert an attractive influence on all the bodies 
belonging to the solar system, which is a proof of its being com- ' 
posed of dense matter. 

Q. How is it known that light and heat emanate from the Sun ? 

A. Both light and heat are perceived when the observer is on 
that side of the Earth which is turned towards the Sun ; for when 
we stand in the sunshine, we not only see the light, but feel the 
heat. 

Q. Is the entire body of the Sun luminous? 

A, No ; its light is supposed to emanate from its outer surface. 



THE SUN. 



31 



Bj means of a telescope, spots may sometimes be seen, which. are 
now presumed to be the dark body of the Sun seen through aper- 
tures in its outer luminous envelope. 

Q. Of what is the luminous surface of the Sun supposed to consist ? 

A. The outer envelope of the Sun is supposed to consist of a 
luminous gas, which the telescope shows to be in motion, and oc- 
casionally parted or broken, so as to reveal the dark body of the 
Sun through the openings. {See Note 8.) 

Q. What is the distance of the Sun from the Earth ? 

A. About ninety-five millions of miles. 

Light, which moves at the rate of about 200,000 miles in a second, requires nearly 
eight minutes and a quarter to travel from the Sun to the Earth ; and a railroad car, 
moving at the rate of thirty miles an hour, would require three hundred and sixty years 
to travel from the Earth to the Sun. 

Q. If the distance of the Sun be so immense, how can it be ascertained ? 

A. By noting the different positions it seems to occupy in the 
heavens, when viewed by two observers on the Earth's surface, 
stationed widely asunder. 

Fig. 31. 




In order to understand how it is that the distance of a remote body may be ascer- 
tained Uy viewing it from ditlcrent yiositions, imagine S (fi(). ol) to be a spider standing 
upon the rim of a wheel, and luoking up at a fly pUiced at F, upon the pane of glas.t 



32 



BOUVIER S FAMILIAR ASTRONOxMY. 



r 



G G, overhead. If the spider were as good a practical mathematician as he is some- 
times believed to be, he would be able to ascertain the distance of the fly without leav- 
ing his wheel. He would first note the direction in which he saw the fly from S ; he 
would then walk along the rim of the wheel to T, its top, and note the direction of the 
fly again. Next, he would continue his walk to V, and observe the direction for the 
third time. Now, when the spider was at T, he would have seen the fly directly over- 
head, projected against the sky as if at A, (the same precise spot, be it observed, against 
which he would have seen him projected if he had been looking at him from C, the 
centre of the wheel ;) but when at S, he would have seen him depicted against the sky 
at R, a spot considerably to the right of A ; and when at V, he would have seen him as 
if at L, considerably to the left of A. The distance between L and R, the extreme 
left and right apparent positions of the fly, would depend upon the length T F, the dis- 
tance of the fly from the wheel; for, it will be observed, that if the fly had been at B in- 
stead of at F, the lines representing the direction in which he was seen from S and V 
would have crossed each other as the lines S D and V E do. The greater the change 
in the apparent position of the fly, caused by the spider's removal through the definite 
distance between S and V, the nearer must be the true place of the fly. The measure 
of the change of apparent position, as from L to R, or from E to D, therefore expresses 
the length of F T, or B T, the distance of the fly from the wheel. 

Man, on the Earth, is a practical mathematician in similar circumstances to the spider 
on the wheel. He moves on the circumference of the Earth, as the spider does on the 
rim of the wheel, and notes the position which the Sun seems to occupy, first as seen 
from one side of the Earth, and then as seen from the other side ; the distance between 
his positions being known, the true distance of the Sun from the Earth may be found 
by an easy mathematical process. When a fourth part of the entire circumference of 
the Earth intervenes between the two positions from which the observation is made, the 
difference of the Sun's apparent place in the heavens amounts to what is called eight 
seconds of space ; that is to the 240 th part of the breadth of the Sun's disc. 

Q. How does the Sun appear when viewed through a telescope ? 

A. Dark spots are often seen on the Sun's disc by the aid of 
the telescope. 

Fig. 32. 




THE SUN. 



33 



Some of the solar spots are of immense size, and as changes are seen to take place in 
them in the space of a few hours, the fluctuations must be very rapid. 

The spot No. 1, taken from drawings made by Sir John Herschel at the Cape of Good 
Hope, contained an area of at least 400,000,000 square miles ; and the black centre of 
that represented in No. 4, was at least 10,000 miles in diameter, which was sufficiently 
large to allow the globe of our Earth to drop throught it, and leave a space of a thou- 
sand miles around it clear of contact. 

The spots are considered by Sir J. Herschel to have an intimate connection with the 
rotation of the Sun upon his axis, which may possibly cause currents analagous to our 
trade-winds J for the spots are seen to occupy zones corresponding to our trade- wind 
regions. 

Q. What APPEARANCE havG these spots through a telescope ? 
A. They are frequently large, and perfectly hlack, surrounded 
by a margin or border less dark than the centre of the spot. 

Q. What is this margin or border called ? 

A, It is called a penumbra, 

Q. Are the spots permanent ? 

A. No ; they appear to enlarge and contract, and often disap- 
pear in a few hours, or even less time. Sometimes they break 
out anew in parts of the surface where there were none. 

Q. Are these spots very extensivi; ? 

A. Some of them are immense, having been known to have a 
diameter of upwards of forty-five thousand miles, and even of a 
much greater extent. 

Q. Is that part of the Sun's disc, unoccupied by spots, uniformly bright ? 

A. No ; its ground is finely mottled with small dots or pores, 
which, when attentively watched through a telescope, are found 
to be in a constant state of change. 

Q. Are there any other peculiarities observable on the surface of the Sun? 

A, Yes ; in the vicinity of large spots strongly-marked curved 
or branching streaks may be observed, more luminous than the rest. 

Q. What are these luminous streaks called ? 

A, They are called faculce, and the black spots are called 
maculce. 

Fig. 33. 



If the centre of the Earth were placed 
at the centre of the Sun, the Sun would 
fill up the whole orbit of the Moon, and 
extend two hundred thousand miles be- 
yond it. Let E {fig. 33) represent the 
Earth, and M the Moon in her orbit, 
which is two hundred and forty thou- 
sand miles from the Earth. The Sun 
would fill up the orbit of the Moon, and 
extend two hundred thousand miles be- 
yond it, to the circle SUN. 




34 bouvier's familiar astronomy. 

Q. What is the supposed thickness of the luminous coating or atmosphere 
of the Sun ? 

A. Sir William Herschel supposed it to be from two to three 
thousand miles in thickness. 

Q. What is the magnitude of the Sun ? 

A. The Sun is eight hundred and eighty-eight thousand miles 
in diameter, or about one hundred and eleven times the diameter 
of our globe. 

Q. What is the form of the Sun ? 

A. It is in the form of a spheroid^ being slightly flattened at 
the poles. 

Q. Is the Sun at rest ? 

A. No ; it revolves upon its axis, as a ball would do if hung 
by a thread and made to spin round. 

Q. How is it KNOWN that the Sun revolves on its axis ? 

A. Because the dark spots seen upon his disc appear to move 
from one edge across to the other edge, and then disappear. 

Q. Why is the Sun believed to be spherical ? 

A. Because the spots may be seen crossing its disc exactly as 
they would if they were marks upon the surface of a revolving hall. 

Q. How long does it take the Sun to complete one revolution on its axis ? 

A. It revolves on its axis once in about tiventy-five days. 

Q. If we know the diameter of two globes, can we estimate their bulks ? 

A. We can ; the bulks or relative contents of two globes of 
unequal magnitudes are to each other as the cubes of their diame- 
ters : that is, their diameters three times multiplied by themselves. 

Q. What proportion does the bulk of the Sun bear to that of the Earth ? 

A. The diameter of the Sun is rather more than one hundred 
and eleven times the diameter of the Earth. Therefore the volume 
or bulk of the Sun must be nearly one million four hundred thou- 
sand times that of the Earth. 

If all the bodies composing the solar system were formed into one globe, it would 
be only about the five hundredth part of the size of the Sun. 



CHAPTER III. 

Interior ftenets, 

' Witi. wliat an awful -world-revolving powex 
Were first the unwieldy planets launched along 
The illimitable void ! There to remain 
Amidst the flux of many thousand years, 
That oft has swept the toiling race of men. 
And all their laboured monuments, away 



INTERIOR PLANETS. 



35 



Firm, unremitting, matchless in their course; 
To the kind-tempered change of night and day, 
And of the seasons, ever stealing round, 
Minutely faithful. Such the all-perfect Hand, 
That poised, impels, and rules the steady •whole. 

Thomson. 

Q. What is meant by intebioe planets ? 

A. Interior, or as they are sometimes called, inferior, planets, 
are those whose orbits are within that of the Earth. 

Q. How MANY interior planets are there, and what are their names ? 

A. There are two interior planets, Mercury and Venus. 

Fig. 34. 



In fig. 34, let S be the place of the 
sun ; M, the orbit of Mercury ; and 
V, that of Venus. It will be seen 
they are both within or interior to 
the orbit of the Earth, E. 




SECTION I. 

Percurg. ? 

Q. What is the name of the planet nearest to the Sun ? 

A, The nearest known planet to the Sun is Mercury. 

•• Mercury, the first. 
Near ordering on the day, with speedy wheel 
FUes swiftest on, infi.aming where he com.es, 
"With sevenfold splendour, all the azure road." 

Q. What is the distance of Mercury from the Sun ? 

A. Mercury is thirty-seven millions of miles from the Sun. 

Q. Is Mercury as large as our Earth ? 

A. No; his diameter does not exceed three thousand miles; 
whereas the Earth is about eight thousand miles in diameter. 

Q. Does Mercury move round the Sun ? 

A. Yes ; he performs one revolution round the Sun in about 
eighty-eight of our days. 



36 



BOUVIER S FAMILIAR ASTRONOMY. 



Q. How long, then, is Mercury's year? 

A. About eighty-eight days, or rather less than three of our 
months. 

Q. What produces the change of day and night? 

A. The revolution of a planet on its axis. 

Q. Does Mercury revolve on his axis ? 

A. It is believed he revolves on his axis once in about twenty- 
four hours. 

The time of his revolution on his axis cannot be ascertained with exactness, ou 
account of his proximity to the Sun. 

Q. How long, then, is Mercury's day ? 

A. Twenty four hours — the same length as our day. 
Q. At what RATE does Mercury move in his orbit round the Sun ? 

A. He travels at the immense rate of one hundred and nine 
thousand miles per hour. 

Q. Is Mercury ever seen on the meridian at midnight ; that is, in that part 
of the heavens directly opposite to the Sun's place ? 

A. No ; Mercury is never seen more than about thirty degrees 
east or west from the Sun. 

Q. Why is Mercury never seen farther from the Sun than thirty degrees ? 

A. Because he revolves round the Sun in an orbit included 
within the orbit of the Earth. 



Fig. 3 




In fig. 35, E represents the Earth in her orbit, and c, 7, d,f, one of the interior pla- 
nets in its orbit within that of the Earth. Let a b represent the concave sphere of the 
fixed stars. When the planet is at c, it is for some time receding in a line from the 
Earth, and, therefore, to us appears stationary. From c to 9, and from g to near d, the 
motion appears direct, or from west to east; when near d, it^s then approaching the 
Earth in an almost straight line, which makes it appear stationary; and from d toy', 
and from/ to n, its motion appears retrograde, or contrary to the order of the signs. 



INTERIOR PLANETS. 37 

Hence, it may be seen, that an inferior planet never appears at a greater angular dis- 
tince from the Sun, among the stars, than from a on one side to b on the other side. 
These ttvo points are called its greatest eastern and western elongations. 

Q As Mercury is so much nearer to the Sun than the Earth, does he re- 
cei'^e more heat? 

A. The heat at the planet Mercury is supposed to be seven 
times greater than on the Earth. 

Mrs. Somerville says, " On Mercury, the mean heat arising from the intensity of the 
Sun's rays, must be above that of boiling quicksilver; and water would boil even at 
his poles." But he may be provided with an atmosphere so constituted as to absorb 
or reflect a great portion of this superabundant heat, so that his inhabitants (if he has 
any) may enjoy a climate as temperate as any on our globe. 

"First, Mercury, amidst full tides of light, 
Rolls next the Sun, through his sraall circle bright; 
Our Earth would blaze heneath so fierce a ray. 
And all its naarble mountains melt away." 

SECTION 11. 

Q. What is the name of that planet situated nearest to the Earth t 

A, It is called Venus. 

" Fairest of stars, last in the train of night ; 
If better thoa belong not to the dawn. 
Sure pledge of day, that crown'st the smiling morn 
"With thy bright circlet, praise Him in thy sphere. 
While day arises, that sweet hour of prime." — Milton. 

Fig. 36, 



Fig. .36 is a telescopic view of Ve- 
nus when only half her illuminated 
di;c is turned towards the Sun. 




Q. Why does Venus shine so brightly ? 

A. Because being nearer to the Sun than the Earth, she 
receives more of his light ; and, being comparatively near to the 



'SS 



BOUVIER S FAMILIAR ASTRONOMY. 



Earth, reflects on us a large portion of the solar light which she 
receives. 

Q. Is the body of Venus opaque, or luminous ? 

A. Venus, as well as the Earth, and all the other planets, is 

opaque. 

Q. If the body of Venus be opaque, how does it shine so brightly ? 

A. Like our moon, she shines by borrowed sunlight. 

Q. How is it KNOWN that Venus bokkows her light from the Sun? 

A. That half of the planet which is towards the Sun is illumi- 
nated, whilst the other half is dark, and, consequently, sheds no 
light. 

Q. When is only half her illuminated disc towards us? 
A. At the time of her greatest elongation. 

Q. What is meant by elongation ? 

A. The difference between the Sun's place and the geocentric 
place of the planet ; that is, the angle formed by lines drawn from 
the centres of the Sun and planet to the centre of the Earth. 




\ 




V [fig. 37) represents the planet Venus in her orbit round the Sun, in which she 
moves through a series of positions, sometimes between the Earth and the Sun, and 
sometimes beyond the Sun. When at a, she is between the Earth, E, and the Sun, S, 
and is then in her inferior conjunction ; and when at c, she is at her greatest western 
elongation, or her greatest angular distance from the Sun towards the west. When at 
h, she is in her superior conjunction ; and at V, she is at her greatest angular distance 
from the Sun towards the east, or at her greatest eastern elongation. When the planet 
is at c, it is seen among the stars at/, and the Sun is seen at the same time at d ; con- 
sequently, the angular distance rf/is the planet's greatest western elongation. In the 



INTERIOR PLANETS. 39 

same way, at V, the planet appears projected on the sphere of the heavens at e / and 
the angular distance from rf to e is its greatest eastern elongation. 

Q. What is meant by conjunction ? 

A. The apparent meeting of the heavenly bodies. 

In the foregoing figure, when the earth is at E, if the planets Mercury or Venus 
should be between the Earth and the Sun, or on the dotted line a S, they would be in 
inferior conjunction. But if either of the above-named planets should be on the dotted 
line h S, that is, beyond the Sun, they would then be in superior conjunction. 

Q. When Venus is between the Earth and the Sun, which body is she nearest to ? 

A. She is nearer to the Earth than to the Sun. 

Q. How far is Venus from the Sun ? 

A. She is about sixty-eight millions of miles from the Sun. 

Q. When Venus is in inferior conjunction, how far is she from our Earth? 

A. When in inferior conjunction she is twenty-seven millions 
of miles from the Earth. 

Q. Does she appear larger when in inferior conjunction than when in supe- 
rior conjunction ? 

A. She does; because when in superior conjunction she is one 

hundred and sixty-three millions of miles from us. 

Q. How much greater does the diameter of Venus appear when in inferior 
than when in superior conjunction? 

A. The diameter of Venus appears more than six times greater 
when in inferior than when in superior conjunction. 

Fig. 38. 




Fig. 38 represents the planet Venus as seen through the telescope when near her 
inferior conjunction. As Venus is usually invisible when in inferior conjunction, that 
is, when between us and the Sun, she appears as a slender crescent only immediately 
he/ore and after her inferior conjunction, proving conclusively that she is a round 
opaque body. 



40 



BOUVIER S FAMILIAR ASTRONOMY. 



Q. Does Venus ever present the appearance of a full orb ? 
A. She does ; when in that part of her orbit beyond the Sun, 
as respects the Earth, she appears with a full round disc. 

Fig. 39. 




Fig. 39 exhibits the phases of Venus as seen through the telescope. At I, the planet 
is represented in inferior conjunction, being between the Earth and the Sun. In this 
position Venus is usually invisible, having her dark side turned towards the Earth. 
She soon appears after this as a morning star, exhibiting a very slender crescent, which 
may be seen by the aid of a telescope. As the planet moves on from east to west, the 
crescent increases, till at N it presents the appearance of a half moon. At this point 
Venus appears to remain stationary for some time, after which she seems to move from 
west to east. This is her direct motion. As she advances through the positions 0, P, 
.and Q, she presents more and more of her enlightened hemisphere, till she arrives at A, 
where she becomes completely full. At A, Venus is in superior conjwictiun, and at her 
greatest distance from the Earth. At B, she appears as an evening star, and exhibits a 
gibbous phase, which increases, till on her arrival at E she again has the appearance of 
a half moon. From E to I she seems to move from east to west, which is called her 
ret-ograde motion. From F she assumes again the figure of a crescent, which diminishes 
in breadth, but increases in extent from horn to horn, until she arrives at I. 
Q. What are the dimensions of the planet Venus ? 

A. Her diameter is about seven thousand seven hundred miles., 
which is but little less than the diameter of our Earth. (Note 9.) 

Q. How LONG is Venus in performing her journey round the Sun ? 

A. She makes one revolution round the Sun in a little more 
than tivo hundred and twenty-four days, or about thirty-two of 
our weeks. 

Q. Then what is the length of Venus's year ? 

A. About thirty -two of our weeks. 

Q. If Venus revolves round the Sun in two hundred and twenty-four days, 
why does she apparently remain on the same side of the Sun for the space of 
more than two hundred and ninety days ? 

A. Because the Earth moves in her orbit in the same direction 
that Venus moves in hers ; but as the earth moves more slowly 
than Venus, she finally outstrips the Earth, thus making her 
synodic revolution, that is, the period between two conjunctions, 
to consist of 584 days. 



INTERIOR PLANETS. 41 

Q. With what velocity does Venus move in her orbit ? 

A. Venus moves at the rate of eighty thousand miles an hour 
in her journey round the Sun. 

Q. Do the planets move with equal velocities ? 

A. No ; the nearer a planet is to the Sun, the greater is its 
orbital velocity. 

Q. Why do those planets near to the Sun move with greater velocity in their 
orbits than those which are more distant ? 

A. Because the nearer a planet is to the Sun, the more power- 
ful is the Sun's attraction ; therefore, it must move with propor- 
tional velocity to overcome that attraction. 

Q. Does Venus revolve upon her axis as she moves round the Sun ? 

A, She does. Venus performs one revolution on her axis in 
about twenty-three hours and a quarter ; therefore the day on 
Venus is very little shorter than ours. 

Q. What causes the variety of seasons ? 

A. The change of seasons is owing to the inclination of the 
axis of a planet to the plane of its orbit. 

Q. Is the axis of Venus inclined to the plane of her orbit ? 

A. Yes ; the axis of Venus is inclined 73J° to the plane of her 
orbit, while that of the Earth is inclined only about 23 J°; so 
that the tropics of Venus are within her polar circles. 

Q. Explain how the tropics of Venus are within her polar circles. 

A. As the axis of Venus is inclined 73J° to the plane of her 
orbit, her tropics must be 73J° on each side of her equator ; that 
is, to within 16J° of her poles ; and as the polar circles are as far 
from the poles as the tropics are from the equator, it follows that 
her polar circles must be within 16J° of her equator. [See Note 10.) 

Q. Where are the tropics situated? 

A. The tropics include that space or part of the sphere called 
the torrid zone ; because the Sun is at one time or other perpen- 
dicular over every part of it, and torrifies or heats it. 

Q. How is it known that the planets Mercury and Venus revolve in orbits 
within the orbit of the Earth ? 

A. Because they are never seen opposite to the Sun ; that is, 
they are never seen in the east when the Sun is in the west ; nor 
are they ever on the meridian at midnight. 

Q. Is there any other proof that these planets revolve in orbits nearer to the 
Sun than that of the Earth ? 

A. Yes ; they each may sometimes be seen to pass ao^oss the 
•Sun's disc like a dark spot. 
• Q. What is the passage of these planets across the Sun's disc called ? 

A, It is called a transit of the planet. 

" Transit" is derived from the Latin word trnusitus, which means passing or going over. 
Q. When is Venus a morning, and when an evening, star? 

A. Venus is a morning star ivom inferior to superior conjunc- 



42 



BOUVIER S FAMILIAR ASTRONOMY. 



tion ; and from superior to inferior conjunction she is an evening 
star, 

"Fair raorning star 
That leads on dawning day- to yonder world 
The seat of Man." 

Q. Has Venus any attendant moon ? 

A. No moon has ever been discovered as belonging to Venus ; 
but owing to her proximity to the Sun it would be very difficult 
to see one, if it even exists. 

Q. Are Mercury and Venus the only planets nearer to the Sun than the Earth ? 

A. No other planets have ever been discovered whose orbits 
are within the orbit of the earth. 



CHAPTER IV. 



"This world 
Poised in the crystal air, with all its seas, 
Mountains, and plains, naajestically rolling 
Around its noiseless axis." 

Q. What is the Earth ? 

A. A glohe enclosed in an envelope of air, called the atmo- 
sphere. 

Fig. 40. 



The atmosphere which surrounds the Earth is not 
thicker in proportion to the bulk of our globe than the 
circular line in the figure, when compared with the space 
which it encloses, or the down on the skin of a peach, 
in comparison with the fruit inside. 



Q. Of WHAT is the large globe of the Earth composed? 

A. It is composed of very small particles of matter^ which at- 
tract each other so strongly, that they cannot be moved about 
among each other without the aid of some force. 

The gravitation of the Earth to the Sun results from the gravitation of all its compo- 
nent parts, which also attract the Sun in proportion to their respective masses. 
Q. Of what is the air which surrounds the Earth composed? 

A. It is a gas formed of extremely minute particles of matter 
floating about freely among each other. 




THE EARTH. 48 

Q. Are the particles which form the air as closkly connected together aa 
those of solid bodies ? 

u4. No ; the particles of gases are not so compact^ but move 
freely among each other. 

Q. How is it known that the atmosphere is a material substance ? 

A, Sensible mechanical effects are produced by its motion and 
its weight. 

Q. What are the mechanical effects produced by its motion ? 

A. The force of the wind drives the sailing ship in its course ; 
for the sails of the ship are as much affected by the material pres- 
sure of the wind, as a ball is, when thrown, by the material pres- 
sure of the hand. 

Q. Is the surface of the Earth smooth ? 

A. No; it is formed into elevations and depressions; but of 

very trifling extent when compared with the globe itself. 

Q. But some mountains are very high, and some mines have been opened to a 
great depth ; do not these produce great irregularity in the Earth's surface ? 

A. The highest mountain on the Earth is only about five miles 
above the level of the sea, and the deepest mine hitherto opened 
does not exceed half a mile ; these irregularities are not greater 
than grains of sand and pin-point scratches on the surface of an 
artificial globe two feet in diameter. 

" These inequalities to us seem, great ; 
But to an eye that comprehends the whole. 
The tumour which to us so monstrous seems 
Is as a grain of sparkling sand, that clings 
To the smooth surface of a sphere of glass ; 
Or as a fly, upon the convex dom.e 
Of a sublime, stupendous edifice." — Lofft. 

Q. Does the atmosphere occupy all the cavities of the Earth's surface ? 

A. The greatest depressions are filled with water, a denser fluid, 
which flows into them, owing to its greater weight, and thus ex- 
cludes a great part of the air. These collections of water are 
called oceans and seas. 

Q. Does any of the water of the ocean ascend into the atmosphere ? 

A. Portions of the water rise into the air as vapor, as far as 
the highest clouds. 

Q. Does any of the atmosphere penetrate into the water ? 

A. Yes ; portions of air are contained in water. It is the va- 
por of the atmosphere which falls as rain. 

Q. Is the Earth at rest ? 

A. No ; it is constantly turning round on its axis with a mi- 
form motion. 

" The globe terrestrial, with its slanting poles, 
And all its pond'rous load, unvrearied rolls." — Blackmors. 



44 BOUVIER'S familiar ASTEONOxMY. 

Q. What is the axis of the Earth ? 

A. An imaginary line passing through the poles upon which 
the Earth is supposed to turn. 

Q. How is it known that the Earth revolves on its axis ? 

A. Because the inhabitants on its surface are, by its rotation, 
carried into the darkness of its own shadow, which produces night; 
and then into the sunlight, which we call day. 

Fig. 41. 
A 





Suppose A B to be the Earth, which is an opaque globe ; S represents the Sun, N the 
deep shadow of midnight, and D the time of noonday. If an observer were to be car- 
ried from D to E, and then to N, he would pass from noonday to midnight. 

Q. How long does the Earth require to perform one complete revolution on 
its axis? 

A. Twenty-three hours, fifty-six minutes, four seconds and one- 
tenth. This is called a sidereal day. 

Q. What is the form of the Earth? 

A. The Earth has the form of a sphere, swelling out in one di- 
rection of its circumference, like an orange. 

Q. How is the Earth known to be spherical ? 

A. Because navigators have sailed round it, without mate- 
rially changing their course; just as a fly may be seen to walk 
round an orange, returning to the place it set out without retrac- 
ing its steps. 

Q. Is there any other proof of the spherical form of the Earth ? 

A. Yes ; the shadow which the Earth casts is only such as be- 
longs to spherical bodies. 

Fig. 42. 




Q. When ships are leaving port, what part of the vessel first disappears ? 
A. The huU first disappears, and finally the topmasts are lost 
to our view as the ship recedes from the shore. 



THE EARTH. 



45 



This is owing to the convexity of the Earth : for although the sea appears level to a 
person standing on the shore, it is sufficiently curved to conceal the hull of a ship while 
the topmasts remain in view. 

"Behold -when the glad ship shoots from the port. 
Upon full sail, the hull first disappears, 
And then the lower, then the higher sails ; 
At length the summit of the towering mast 
Alone is seen ; nor less, when from the ship 
The longing sailor's eye, in hope of shore; 
For then, from the topmast, though more remote 
Than either deck, the shore is first beheld." 

Q. What are the points at the extremities of the Earth's axis called ? 
A. They are the poles. 

Q. Is the Earth completely spherical ? 

A, No ; it is an ohlate spheroid, that is, in the shape of an 
orange, being a little flattened at the poles. 

Q. What part of the Earth protrudes the most ? 

A, The equatorial regions, or that part at and near the equator. 

"Q. Why are the equatorial regions more protuberant than any other portions 
of the Earth ? 

A. Matter is heaped up there by the force of its rapid motion 
on its axis. (^See Note 11.) 

Q. When a wet grindstone is made to revolve quickly, why does the water fly 
OFF from its edge ? 

A. Because the rapid motion of the stone increases the centri- 
fugal force. 

Q. If the motion of the grindstone be less rapid, will the water fly off ? 

A. No; it will be heaped up on its edge, instead of being 
thrown off. 

Q. To what extent does the equatorial portion of the Earth protrude ? 

A. The breadth of the equatorial diameter is to the breadth 
of the polar diameter as 299 to 298. 

Q. What, then, is the difference in miles between the two diameters ? 

A. The equatorial diameter is twenty-six miles and a half 
longer than the polar diameter. 

Q. How is the amount of this difference ascertained ? 

A. This difference is ascertained by means of the pendulum. 
The pendulum beats faster in proportion to its proximity to the 
Earth's centre ; and the rate of its beating when near the poles 
shows that it is there the one two hundred and ninety -eighth part 
nearer the Earth's centre than it is at the equator. 

Q. Could the difference between the polar and equatorial diameters be dis- 
cerned by the eye, if the Earth could be viewed from the Moon ? 

A. No ; from that distance the Earth would appear to have a 
completely circular outline, the difference between its polar and 
equatorial diameters being too insignificant to be detected by 
the eye. 



> 




46 botjvier's familiar astronomy. 

Q. What is the real size of the Earth ? 

A, The Earth is about eiglit thousand miles in diameter. 

More correctly, it is 7926 miles in diameter. 

" Of this dependent universe, our planet is a part so small, that no arithmetician can 
assign a fraction low enough to express its proportion to the whole." 
Q. Row is the exact size of the Earth ascertained ? 

A. By measuring small and known proportional parts. 

Fig. 43. 



Let the space hetween A B (fig. 43) be known to be a twenty- 
fourth part of the circumference of the Earth ; and let it be 
found to measure one thousand miles. Then the whole cir- 
cumference would necessarily measure twenty-four thousand 
miles j for it contains twenty -four such intervals. 



Q. Has a t"wenty-fourth part of the Earth's circumference ever been meastteed ? 
A. In India, rather more than a twenty-fourth part of the 
Earth's circumference has been measured. 

Q. Has the Earth any other motion than that of its revolution on its axis, 
producing day and night ? 

A. Yes ; it revolves round the Sun once in a year, 

Q. In the annual revolution round the Sun, does the Earth's axis always retain 
the SAME position ? 

A. It does ; the axis of the Earth always points towards the 
same fixed star. 

This problem may easily be solved by observing that any fixed object on the Earth, 
as the index or gnomon of a sun-dial, always points to the same part or spot in the 
heavens ; whereas, if the position of the Earth's axis were changeable, it would point 
to different parts of the heavens in the different seasons of the year. 
Q. What is the distance of the Earth from the Sun ? 

A. About ninety-five millions of 7niles. 

Q. Does the Earth always keep at the same distance from the Sun ? 

A. No ; the Earth is nearer to the Sun at one part of the year 

than at any other. 

Q. How is it KNOWN that the Earth is nearer to the Sun at one part of the year 
than at any other ? 

A. Because the Sun appears larger sometimes than at others, 
and all distant bodies appear of a larger size when brought nearer 
to the eye. 

Q. When has the Sun the largest apparent size ? 

A. About the beginning of December. 

Q. When has the Sun the smallest apparent size ? 

A. About Xh.Q first of July ; and as it is not probable that the 
Sun changes his real size periodically, the observed change of his 
apparent size can only arise from an actual change of distance. 



THE EARTH. 47 

Q. How can the exact breadth of the Sun's disc be ascertained ? 
A. By observing the time it requires to pass across a thread 
placed perpendicularly in the focus of the eye-glass of a telescope. 

Q. How much farther is the Earth from the Sun in July than in December ? 

A. Nearly three millions of miles farther from the Sun in July 
than December. 

Q. What is the form of the path which the Earth describes round the Sun ? 

A, Its form is that of an ellipse. {See Note 12.) 
Q. In what part of the Earth's orbit is the Sun placed ? 

A. The Sun occupies one of the foci. 

Fig. 44. 
B 




Let A B P C represent the orbit of the Earth, and S the Sun. It will be seen that the 
Sun is not in the centre of the elliptical orbit, but in one of the foci. It must be remem- 
bered that the ellipse delineated in the figure is not sufficiently circular to represent the 
orbit of the Earth, which is so slightly elliptical that, were it drawn in true proportions, 
its ellipticity would not be discernible by the eye. 

Q. What is meant by perihelion ? 

A. That point in a planet's orbit nearest to the Sun. 

Q. What is meant by aphelion ? 

A. That point in a planet's orhit farthest from the Sun. 

Q. When the Earth is approaching its perihelion, why does it not pursue its 
COURSE in a straight line till it comes in actual contact with the Sun? 

A. Because as the Earth approaches the Sun his attraction is 
mere poiverful, and thus causes it to move faster. 

Q. How does the increased motion of the Earth prevent it from coming in 
contact with the Sun ? 

A. By conferring upon it increased momentum, its projectile or 
centrifugal force is increased. 

When the Earth is at the point B, {jig. 44,) its velocity increases until it arrives at 
its perihelion P. At this point its increased momentum overcomes the Sun's attraction, 
and it pursues its course through C to A, its aphelion, where its motion is the slowest. 
At A the Sun's attraction overcomes the projectile force, and it is drawn again towards 
that luminary with increased momentum, till it arrives at its perihelion at P. 



48 



BOUVIER S FAMILIAR ASTRONOMY. 



Q. Why does the Earth begin to approach the Sun when it has arrived at 
aphelion, or that point in its orbit farthest from that luminary ? 

A, Because then the Sun's attraction preponderates over its 
momentum. 

Q. Why does the Earth begin to recede from the Sun when it has reached its 
perihelion, or that point in its orbit nearest to that luminary? 

A. Because then its momentum overcomes the Sun's attraction. 

Q. How much faster does the Earth move when in perihelion than when it 
is in aphelion ? 

A. It moves so much faster in perihelion, that a line drawn 
connecting the centre of the Earth with the centre of the Sun, 
would pass through the same quantity of superficial space, and in 
the same time, that another line would do, drawn from the centre 
of each, when the Earth is in aphelion. 

Fig. 45. 




In the above figure let S a represent a line connecting the centres of the Earth and 
Sun when the Earth is at perihelion, or her least distance from the Sun ; and let S b 
represent a line connecting their centres when the Earth is at aphelion, or her greatest 
distance from the Sun ; then if the contents of the angular space b S d were exactly 
equal to the contents of the difi"erently shaped angular space a S c, then b d would re- 
present the portion of its orbit through which the Earth would travel in some given 
time, say a month, when at its remotest distance, and a c the portion of its orbit through 
which it would travel in a month when at its nearest distance ; for the long line S b 
and the short line S a both sweep over the same superficial space or area, in the same 
given time. The Earth, therefore, moves much faster from a to c than from b to d. 
{See Note 13.) 

Q. Why does the Earth move round the Sun ? 

A. Because it is impelled hj two forces, which, combined to- 
gether, drive it round the Sun. 

Q. What are these two forces called ? 

A. The projectile, or centrifugal force belonging to the Earth 
is that which impels the earth to fly from her orbit ; and the attrac- 
tion of gravitation, or centripetal force, which is a power seated in 
the Sun, is that which gives the Earth the tendency to approach 
the Sun. Thus the two forces combined retain the Earth in her 
orbit, and cause it to move round the Sun. 



THE EARTH. 



49 



Q. In what direction does the power seated in the Stin influence the Earth ? 
A. The solar power draws the Earth towards the Sun. The 
Sun's mass attracts the Earth's mass. 

''Mass" means the entire amount of matter contained in a body. 
Q. What is that power called which is seated in the Sun's mass ? 

A. It is called centripetal force, or the attraction of gravitation. 

It is a universal law, that all ponderable bodies have a mutual attraction in propor- 
tion to their mass. 

Q. Does the Earth exercise an attractive influence over other bodies? 

A. It does. The Earth attracts the Sun and Moon, as well 
as suffers attraction by them. It also attracts all bodies on its 
surface. 

Q. How is the Earth's attractive influence most plainly shown ? 

A. By its retaining small bodies upon its surface with a cer- 
tain amount of force. 

Q. Why cannot we lift a stone or other body without exerting some force ? 

A. Because we must overcome the resistance caused by the 
attraction of the Earth's mass for the mass of the stone. 

Q. What is this resistance called ? 

A. It is called weight or gravity/. 

Q. Why does not the Earth fall towards the Sun, as a stone would fall to- 
wards the Earth ? 

A. Because the influence of that power called the centrifugal 
force, prevents the Earth from falling to the Sun. 

Q. What is the nature of that power called centrifugal force ? 

A. It is the moving or projectile power, acting upon the Earth 
in a different direction from the solar influence. 

The moving power which a ball possesses, after it has been thrown or struck by the 
hand, is its projectile force, 

Q. In what direction does the projectile force of the Earth impel it ? 

A. It impels it in a direction very nearly at right angles with 
that line along which the Sun's attraction acts. 

Fig. 46. 



Suppose S {fig. 46) to be the Sun, and 1 the 
Earth in one part of her orbit. Then the Earth's 
projectile or centrifugal force impels it towards M, 
along the line I M; and this crosses at right an- 
gles the line 1 S, which represents the direction 
from which the Sun's attraction acts. The pri- 
mary impulse which made the Earth a moving 
body must have been exerted in this way, directly 
at right angles with the line leading from the 
Eartix to the Sun. 




} 



50 bouvier's familiar astronomy. 

Q. When two forces impel a body in different directions, what course does 
the moving body pursue ? 

A. It moves in a line intermediate to those directions. It 

obeys both forces by taking a middle course between them. 

Q. Since the Sun attracts the Earth towards itself, while the Earth's projectile 
force impels it in a direction at right angles with the line leading towards the 
Sun, in what direction does the Earth really move ? 

A. It obeys both forces, and moves in a direction intermediate 
to the lines in which they act. 

In fig. 46, page 49, suppose the Earth to be at 1 in her orbit, and to receive an im- 
pulse in the line 1 M, at the same time that she is drawn towards the Sun S, in the line 
1 S ; she would in that case take the intermediate course between them, and arrive at 2. 
Q. Does the Earth move in a straight line under the combined influences of 
the two forces ? 

A. No ; it moves in a curved line everywhere surrounding the 
Sun. 

Q. Why does the Earth move round the Sun in a curved path ? 

A. Because the momentum of its own motion, and the attrac- 
tion of the Sun, are continued forces, constantly acting in new 
dii'ections in consequence of the Earth's motion in her orbit. 

The curved line is the only one that can be always intermediate to the new directions of 
these shifting forces. Thus, in fig. 46, the lines 1 M, 2 M, 3 M, show the altered directions 
in which the Earth's projectile force acts as it moves round in its curve from 1 to 2 and 3, 
while the lines 1 S, 2 S, 3 S, show the altered directions of the Sun's attraction during the 
progress. The circular line, 1, 2, 3, shows how the actual path of the Earth is curved, in 
obedience to the change of the direction of the two impelling forces. The same thing oc- 
curs when a sling is made to whirl round the head. The projectile force of the whirling 
stone always inclines it to fly off in some line corresponding to 1 M, 2 M, 3 M of the 
figure ; while the string always holds it in some line corresponding to 1 S, 2 S, 3 S. The 
path pursued is the circular one, which is always intermediate to the shifting directions 
in which the two forces act: and the moment the stone is freed from the influence of 
one of these forces, the holding-in power of the string for instance, it obeys the other 
force, and flies off in a direction at right angles to the string. 
Q. What is meant by momentum ? 

A. The onward impulse, or force of motion of a body. 

Q. From what source does the Earth get its momentum ? 

A, The Earth has received its motion from the hand of the 
Creator. Its momentum is dependent on the velocity with which 
it was launched into space. 

The case is the same, whether the Earth's motion was primarily communicated to it 
by the Creator, or was transferred therefrom by some intermediate agency. The same 
conclusion is arrived at in either case. It does not matter whether the ball is said to be 
impelled by a bat, or by the hand, because in both instances it is understood that the 
motion is given to the ball by the player. 

Q. To what is the momentum of the Earth's motion due ? 

A. The momentum of the Earth's motion is due to the amount 
of its mass, as well as to the velocity of its motion. 

Q. What is the velocity with which the Earth moves in her orbit? 

A. It moves at the rate of about sixty-eight thousand miles an 
hour. 

Q. How is it KNOWN that the Earth moves with this velocity ? 

A. The Earth's orbit is a curve, whose mean distance from the 



EXTERIOR PLANETS. 



51 




Sun is about ninety-five millions of miles. It revolves in this 
orbit in three hundred and sixty-five days ; therefore its hourly 
motion may easily be found. 

In the annexed figure, let A B C D represent the Fig. 47. 

orbit of the Earth, which is not, however, a complete 
circle. The line S E will indicate the distance of 
the Earth from the Sun. Now, as S E, which is 
ninety -five millions of miles, is only one -half of the 
diameter of the Earth's orbit, the whole diameter 
must be twice ninety-five millions of miles, or one hun- 
dred and ninety millions. The diameter F E, there- 
fore, repre-ents one hundred and ninety millions of 
miles. The circumference of a circle is equal to its 
diameter multiplied by 3*14. Consequently, the cir- B} 
cumference of the Earth's orbit is a little more than 
five hundred and ninety-six millions of miles. At an 
even rate of sixty-eight thousand miles per hour, 
the Earth would accomplish this journey in eight 
thousand seven hundred and sixty-four hours ; and 
there are eight thousand seven hundred and sixty- 
six hours in a year. This calculation is deemed 
sufficiently accurate to elucidate the subject without 
taking any fractions into the account. 

Q. What would be the result of increasing the velocity of the Earth's motion ? 

A. The Earth would move farther from the Sun under the in- 
fluence of the increased momentum. 

Q. What would be the result of diminishing the velocity of the Earth's motion ? 

A. The Earth would move nearer and nearer to the Sun, be- 
cause the diminished momentum would not then be sufficient to 
prevent the Sun from attracting the Earth to itself. 

Q. Why is it, then, that the Earth moves evenly in its curved orbit, neither 
falling towards the Sun, nor flying away from it ? 

A. It is because the Earth's mass, and the velocity and direc- 
tion of the projectile force primarily impressed upon it, are so ex- 
actly adapted to the Sun's mass and attractive power, that neither 
the momentum of the Earth, nor the attraction of the Sun, can 
ever neutralize or overbalance each other. 

Thus wonderfully are all the planets sustained in space. The still more enormous 
mass of the Sun attracts them ; but they are already moving so fast that their own mo- 
mentum prevents them from obeying the Sun's attraction. The attraction of the Sun, 
therefore, merely causes the Earth to move in a curved line forever returning into itself, 
or very nearly so ; and that curved line becomes its fixed orbit. 



CHAPTER V. 

••Give me the ways of wandering stars to know, 
The depths of Heaven above, and Earth below." 
Q. What are the exterior planets ? 

A. They are those planets whose orbits are exterior or outside 
of the orbit of the Earth. 



52 



BOUVIBR S FAMILIAR ASTRONOMY. 



Q. How is it knoTTO that the orbits of these planets are extekior to the orbit 
of the Earth ? 

A, Because they are never seen to cross the Sun's disc, as 
Mercury and Venus do ; and they may be seen at times in that 
part of the heavens directly opposite to the Sun ; that is, when 
the Sun is setting, they may sometimes be seen rising, or they 
may be seen sometimes on the meridian at midnight. 

Fig. 48. 




Let E represent the Earth, a d b e g the concave surface of the heavens sarrounding 
it, M the position of Mars in its orbit surrounding that of the Earth E, and S the posi- 
tion of the Sun. From the Earth, Mars is seen among the fixed stars at a, and the Sun 
at h, points which are directly opposite to each other; consequently, Mars is then 
on the meridian at midnight. Now, let V represent Venus in her orbit, surrounded by 
the Earth's orbit ; it is plain that the Sun and Venus can never be seen in opposite 
parts of the^heavens. They would appear together at b, both when the planet Venus is 
at V and c in its orbit, and at all other times it would appear to oscillate with regard 
to the Sun's place, never, however, getting farther from him than the distance from b to 
d, or from b to e; therefore, Venus can never be seen on the meridian at midnight. 
Q. What are the names of the exterior planets ? 

A. Mars, the Asteroids, Jupiter, Saturn, Uranus, and Nep- 
tune. 



EXTERIOR PLANETS. 



53 



SECTION I. 

Pars, d" 

Q. What is the name of that planet fourth in order from the Sun, and aext 
beyoni the orbit of the Earth ? 

A, It is known as the planet Mars. 

"See Mars, alone, runs his appointed race, 
And measures out exact the distant space; 
Nor nearer does he -wind nor farther stray. 
But finds the point -where first he rolled away." — Bakeh. 
Q. Explain why Mars may be sometimes seen on the meridian at midnight. 

A. As his orbit is exterior to that of the Earth, he may some- 
times be opposite to the Sun ; that is, on the meridian at mid- 
night, or rising in the east when the Sun is setting in the west. 

Q. Can you explain why a planet is opposite to the Sun when it is on the meri- 
dian at midnight? 

A. Yes ; when the Sun is on the meridian, it is noon, and the 
point immediately opposite to it must be the point of midnight. 

Fig. 49. 





Thus, to a spectator situated at N, (Jig. 49,) it would be noon ; and when he would be 
carried by the Earth's piotion on her axis round to M, it would be midnight, in which 
situation the point overhead would be opposite to the Sun. 

Q. How long does Mars require to perform one revolution round the Sun? 

A, Mars revolves round the Sun in about six hundred and 
eighty-seven days, or very nearly twenty-three of our months. 

Q. What constitutes the length of a planet's year ? 

A. The time it requires to perform one revolution in its orbit. 

Q. How LONG, then, is the year of Mars ? 

A. Twe7ity-three months, or nearly two of our years. 

Q. What is the distance of Mars from the Sun ? 

A. Mars revolves about the Sun at a mean distance of one 
hundred and forty -two millions of miles. 

Q. What is the distance of Mars from the Earth ? 

A. Mars is at various distances from the Earth in various parts 



54 bouvier's familiar astronomy. 

of his orbit. Sometimes he is only about fifty millions of miles 
from us, and at others as far distant as two hundred and forty 
millions of miles. 

Q. Explain this. 

A. When Mars is at M, {fig. 48,) and the Earth at E, the two 
bodies are only separated by the space of a little less than fifty 
millions of miles ; because the Earth is ninety-five millions of miles 
from the Sun, and Mars is one hundred and forty-two millions of 
miles distant from him. But when he is at/, and the Earth at E, 
the two bodies are separated by the interval E/, which is greater 
than the distance M E by the entire breadth of the Earth's orbit, 
which is one hundred and ninety millions of miles. So that when 
Mars is at /, and the Earth at E, he is two hundred and forty 
millions of miles from the Earth. 

Q. Does Mars always appear of the same size ? 

A. No ; when opposite to the Sun, as at M, [fig. 48,) he ap- 
pears much larger than when he is at/, beyond the Sun.* 

Q. Does he appear bkightee "when he appears the largest ? 

A. Yes ; when in opposition, that is, when he is on the meri- 
dian at midnight, he appears much brighter, because he is then 
nearer to us. 

Q. Does Mars keturn in opposition to the Sun again in six hundred and eighty- 
seven days, which is the time he requires to perform one revolution round that 
luminary ? 

J.. No ; for while Mars goes once completely round in his orbit, 
the Earth performs one revolution round the Sun, and the greater 
part of another. 

Q. How MANY DATS, then, are required for Mars to be in opposition to the Sun? 

A, About seven hundred and eighty days are required for 
Mars to be again in opposition, in which time the Earth will have 
performed rather more than two revolutions round the Sun. 

Q. What is the reai size of the planet Mars ? 

A. Mars is about /owr thousand one hundred miles in diameter ; 
which is but little more than half the diameter of the Earth ? 

Q. Does Mars assume the same variety of phases as Mercury and Venus ? 

A. No; Mars never appears like a crescent, because he can 
never be so placed with regard to the Earth and Sun that less 
than half of his illuminated hemisphere is turned towards the 
Earth. 

When Mars is at M, {fig. 48,) and the Earth at E, his illuminated hemisphere- is 
turned towards the Earth as well as towards the Sun. But when Mars is at h, a little 
more than half the illuminated hemisphere is turned towards the Earth, and the planet 
appears gibbous ; that is, it has one of its illuminated edges flatter than the other, like 
the Moon three or four days before and after the full. 



The greatest and least apparent diameters of Mars are 18" and 4" of angular mea- 
ment. 



Burement 



EXTERIOR PLANETS. 55 

Q. How does Mars appear when viewed through the telescope ? 

A. When Mars is viewed through a telescope at the time of 
his nearest approach to the Earth, his disc is found to be varie- 
gated bj patches of different shades of color and brilliancy. His 
general appearance is that of a dusky red color. 

Fis:. 60. 




Q. Have any other peculiarities of Mars been revealed by the telescope ? 

A. Brilliant white spots surround each of his poles when it is 
again turned towards the Sun, after having been a long time de- 
prived of his beams. These spots diminish gradually under the 
influence of the solar heat. 

Q. What are these white spots supposed to be ? 

A. They are supposed to be owing to an accumulation of snow. 

Cassini and Sir William Herschel believed they could detect indications of the pre- 
sence of a dense atmosphere round the planet Mars, but more recent observations, and 
particularly those of Sir James South, disprove its existence. 
Q. Does Mars revolve on his axis ? 

A. Yes ; Mars revolves on his axis in a little more than twenty- 
four hours and a half, which makes his day only half an hour 
longer than ours. 

Q. Is the axis of Mars, like that of our Earth, inclined to the plane of his 
orbit ? 

A, Yes ; the axis of Mars is inclined 30° 18' to the plane of 
his orbit. 

Q. What EFFECT is produced on a planet by the inclination of its axis ? 

A. The inclination of a planet's axis causes the variety of 
seasons. 

Q. Are the seasons of Mars similar to those on the Earth ? 

A. They are ; because his axis has nearly the same inclination 
to his orbit as that of the Earth. 



56 



BOUVIER S FAMILIAR ASTRONOMY. 



SECTION II. 

Astronomers are now agreed to aflSx to each of these planets the sign of a small circle, 
with a number within it, in the order of their discovery. 

"There's not tlie smallest orL -wiiieh. thou behold'st 
But in his motion like an angel sings, 
Still quiiing to the young-eyed cheruhims. 
Such, harmony is in ircmortal sotiIs ; 
But whilst this muddy vesture of decay 
Doth, grossly close it in, we cannot hear it." — MEBCHAirr of Vsktcb. 

Q. What is the next planet beyond Mars ? 

A. There is a series of very small planets, which revolve in 
orbits not very distant from each other, and beyond the orbit of 
the planet Mars. 

Q. What are these small planets called ? 

A. They are called Asteroids, or Ultra Zodiacal Planets. 

Q. How MANY Asteroids are there, and what are their names ? 

A. The number of Asteroids now known amounts to eighty- 
one. Their names, the dates of their discovery, and discoverers, 
are as follows : 









Period of Sidereal 


Name. 


Date of Discovery. 


Discoverer. 


Revolution, 
in days. 


1. Ceres 


1801, Januarr 1... 


Piazzi 


1681. 


2. Pallas K 


1802, March 28.... 
1804, September 1 


Olbers 


16'^ 


3. Juno 


Harding, Bremen 


1592. 




1807, March 29. . . . 
1845, Decembers.. 
1847. July 1 






5. Astraea 


Hencke, Driesen, (Prussia) 


1511. 


6. Hebe 


Hencke, Drieseo, (Prussia) 


1380. 




1847, August 13... 

1847, October 18... 

1848. April 25 












9. Metis 


Graham, Sligo 


1347. 


10. Hvgieia 


1849, April 12 


De Gasparis, N aples 


2043. 


11. Parthenope 


1850, May 11 


De Gasparis, Naples 


1402. 


12. Victoria 


1850, September 13 


Hind, London 


1301. 


1.3. Egeria 


1850, Novimber .2. 


De Gasparis, Naples 


1511. 


U. Irene 


1S51, Mayl9 


Hind, Loudon 


1518. 


15. Eunomia 


1851, July 29 


De Gasparis, Naples 


1570. 


16. Psvche 


1852. March 17.... 


De Gasparis, Naples 


1826. 


17. Thetis 


1852, April 17 


Luther, Bilk (DusseldorO 


1420. 




1852, June 24 


Hind, London 


1270. 




1852, August 22.... 
1852, September 19 






20. Mas.«alia 


De Gasparis, Naples 


1366. 


21 Lutetia 


1852, November 15 
1852, November 16 


Goldschmidt, Paris 


1388. 


22. Calliope. 




1812. 




1852, December 15 

1853, Aprils 

1853, April 7 

1853, Mav 5 


Hind. London 

De Gasparis. Naples 

Chacornac. Marseille' 




2i. Themis 


2034. 


2.T Phocea 


1359 


26. Proserpine 


Luther. Bilk 


1581. 


27. Euterpe 


1853, November 8. 


Hind, London 


1314. 


2H Bellona 


1854, March 1 

1854. March 1 


Luther, Bilk 


1689. 


23. Amphitrite 


Marth. London 


1492. 


.SO. Urania 


1854, July 22 


Hind, London 


1329. 


.31. Enphrosyne 


1854. September 1. 


Fergu.son . Washington 


2048. 




1854. October 26... 
1854, October 28... 






33. Polvhvmnia.... 


Chacornac, Paris 


1778. 


34. Circe 


1855, April 6 




1609. 


3.=). Lencothea 


1855, April 19 


Luther. Bilk 


1908. 


36 Atalanta. 


1855. Octobers.... 
1855. Octobers.... 


Goldschmidt, Paris 


1666. 


37. FideB 


Luther. Bilk.... 


1569. 


3R. Leda 


1657. 


39. Laetiti.1 


ia56, Februarys.. 


Chacornac, Paris 


1684, 


40. Harmonia 


1856, March 31.... 


Goldschmidt, Paris 


1247. 


41. Daphne 


1856, Mav 22 


Goldschmidt, Paris 


1779. 


42. Isis 


1856, Mav 23 


Pogson . Oxford 


1392. 


43. Ariadne 


1857, April 15 


Popson. Oxford 


1195. 




1857, Mav 27 

18=i7.June27 






4.S. Eueenia 


Goldschmidt. Pari.s 


1640. 


46. Hestia 


1857, August 16.... 


Pogsou, 0.\foru 


1470. 



PLANETOIDS — PLANETS. 



57 



Name. 


Date of Discovery. 


47. Melete 


1857, September 9. 


48 Aglaia . 


1857. September 15 
1857, September 19 


49. Doris 


f.». Pales 


1857, September 19 


51. Virginia 


1857, October 4.... 


52. Nemausa 


1858, January 22.. 


53. Kuropa 


1858, February 6.. 


54. Calypso 


1858, April 4 


5.->. Alexandra 


1858. September 10 


56. Pandora 


1858, September 10 


57. Mnemosyne 


1859, September 22 


5s. Concordia 


1860, March 24.... 


59. Olympia 


1860, September 12 


60. Echo' 


1860, September 15 


61. Danae 


1860, September 19 




1860, September 14 

1861, February 10. 


63. Ausonia 


64. Angelina 


1861, March 4 


63. Maximiliana.... 


1861, March 8 


66. Maia 


1861, April 9 


67. Asia 


1861, April 17 

1861, April 29 


68. Hesperia 


69. Leto 


1861, April 29 


70. Panopea 


1861, May 5 


71. Niobe 


1861, August 13... 


72. Feronia 


1862, February 12. 


73. Clytie 


1862, April 7 


74. Galatea 


1862, August 29.... 


75. Eurydice 


1862, September 22 


76. Freya 


1862, October 21... 


77. Frigga 


1862, November 12 


78. Diana 




79. Eurvnome 


1863, September 19 


80. Sappho 


1864, May 3 


81. Terpsichore 


1864, September 30 



Discoverer. 


Period of Sidereal 
Kerolution, 
iu days. 


Goldschmidt (Paris), & Schubert (St. Petersburg) 
Luther, Bilk 


1529. 

1788. 


Goldschmidt Paris... . 


1998. 












1.3;j0. 


Goldschmidt, Paris 

Luther, Bilk 


1993. 
1543. 


Gold'chmidt Paris 


1629. 




1674 


Luther, Bilk 


2049. 


Luther Bilk 


1619. 




16;«. 


Ferguson , Washington 

Goldschmidt, Paris 


1352. 
1902. 


De Gasparis, Naples 


1356. 
1601. 








1588. 


Pogson, Madras 

Schiaparelli, Milan 

Luther Bilk 


1375. 
1893. 
1688. 






Luther Bilk 


1671. 


Peters (Clinton, New York). & Safford(Washington) 

Tuttle, Cambridge, Massachusetts 

Tempel, Marseilles 


1148. 
1590. 
1509. 






D' Arrest, Copenhagen 

Peters, Clinton, New York 


2080. 
1360. 


Watson, Ann Arbor, America 


Not determined. 


Tempel, Marseilles 


Not determined. 



Q. In what way do the Asteroids differ from the other members of the pla- 
netary system ? 

A. Thej differ— 

1. In point of size; the largest of them being only a few 
hundred miles in diameter ; and 

2. Their orbits are generally much more inclined to the plane 
of the ecliptic than the other planets. 

Q. What is the distance of the Asteroids from the Sun ? 

A. They are all situated at the distance of from two hundred 
to three hundred millions of miles from the Sun, and between the 
orbits of Mars and Jupiter. (See Note 14.) 

They are situated in a belt or zone only about one hundred millions of miles in width. 

Q. What is the form of the orbits in which the Asteroids revolve round the 
Sun? 

A. They revolve in ellipses. (See Note 15.) 

Q. Are the Asteroids visible to the naked eye ? 

A. The greater part of them are invisible without the aid of 
a telescope. 

Q. In what part of the heavens are the planetary bodies to be seen ? 

A. All the planets, except some of the Asteroids, are to be 
found in the Zodiac, a belt or zone extending 8° on each side of 
the ecliptic. 

Q. Why are some of the Asteroids found beyond the limits of the Zodiac ? 

A. Because their orbits are so much inclined to the orbit of 
the Earth. 

Q. How LONG are the Asteroids in performing a journey round the Sun ? 

A. As far as is now known, they perform their revolutions in 
from about three to five years. 



58 



BOUViER s Familiar astronomy. 



SECTION III. 
• |«piler. % 

"More yet remote from, day's all-cheering source. 
Vast Jupiter performs his constant course ; 
Four friendly moons, -with borrowed lustre rise, 
Besto-w their beams benign, and light his siies." 
Q. What is the next plaket beyond the Asteroids ? 

A. The planet Jupiter. 

Q. How is it KNowx that Jupiter is farther from the Sun than Mars ? 
A. The illuminated disc of Jupiter never perceptibly assumes 
the gibbous form which may be observed in the planet Mars. 

The orbit in -which Jupiter moves is so much more vast than that of our Earth, that 
the very trifling portion of the planet's hemisphere which is turned away from the Sun 
can never be distinguished by a spectator on the Earth. 

Fig. 51. 




In the accompanying figure, let E represent the Earth in her orbit, and J the planet 
Jupiter. It will be seen that the illuminated half of Jupiter's sphere will always be 
turned towards us in every part of his orbit; consequently, he can never present the 
various phases of Venus, nor appear gibbous like Mars. 
Q. How FAR is Jupiter from the Sun? 

A. Jupiter is about four hundred and ninety-four millions of 
miles from the Sun. 

Q. As Jupiter is so much more distant than Mars, how is it that he appears 
so much more bright and conspicuous ? 

A. Because Jupiter is a much larger body than Mars. 

Jupiter's diameter is more than twenty times greater than the diameter of Mars ; 
although so much more remote, he always presents a disc nearly six times greater than 
the mean apparent diameter of Mars. 

Q. What is the real size of Jupiter ? 

A. Jupiter, the largest planet of our system, is about eighty^ 
nine thousand miles in diameter. 



EXTERIOR PLANETS. 



59 




It would require nearly 1300 globes of the size of our Earth to make one of the bulk 
of Jupiter, A railway carriage, travelling at the rate of 33 miles an hour, would travel 
round the Earth in a month, but would require more than eleven months to perform a 
journey round Jupiter ! 

Q. How long does it take Jupiter to make one revolution round the Sun ? 

A. Jupiter moves round the Sun in a little less than twelve 
years ; or, more properly, in eleven years, ten months, and seven- 
teen days. 

Jupiter is seen in that part of the heavens which 
is opposite to the Sun (that is, on the meridian at 
midnight) once in every interval of thirteen months 
and nearly three days. This is becaiise when the 
Earth has completed one revolution, Jupiter has ad- 
vanced only a twelfth part toward the completion 
of his journey round the Sun. In the diagram, S 
represents the Sun, a the Earth in her orbit, and b 
Jupiter in his orbit, on a given day, say the first of 
January. It will be seen that Jupiter is directly 
opposite to the Sun with regard to the Earth. Now, 
on the first of January of the following year, the 
Earth will have arrived again at a, after having 
completed one revolution ; but as Jupiter requires 
nearly twelve years to perform his journey round 
the Sun, he will only have advanced one-twelfth part 
of the circle in that time ; that is, from b to d. But 
as the Earth passes over the twelfth part of her 

orbit in one month, at the expiration of a year and one month, that is, thirteen months, 
she will have arrived again at c, which is in the same situation with regard to Jupiter 
that she occupied when at o. Of course this is not an exact calculation, as the days, 
and fractions of days, are not taken into the account. 

Q. Does Jupiter rotate on his axis as he advances in his orbit round the Sun? 

A. Jupiter rotates on his axis once in 7iine hours and fifty-six 
minutes. 

Q. What is, then, the length of a day and night on the planet Jupiter? 

A. Nearly ten hours, or, more properly, nine hours and fifty- 
six minutes. 

Q. How long, then, do its inhabitants enjoy the light of the Sun? that is, 
how long is it from sunrise to sunset ? 

A. Not quite five hours. 

On account of the vast size of Jupiter, all places at his equator are carried round by 
his diurnal motion at the rate of four hundred and sixty-five miles per minute. On the 
equatorial surface of the Earth, the diurnal motion is only about seventeen miles in a 
minute. 

Q. Is Jupiter a perfect sphere ? 

A. No ; Jupiter, like the Earth, is not a perfect sphere, but 
flattened at the poles. 

Q. What CAUSES Jupiter to be flatter at the poles than at the equator ? 

A. The great velocity with which he revolves on his axis. 

Q. What is the difference between the equatorial and polar diameters of 
Jupiter ? 

A. Jupiter measures nearly six thousand miles more through 
his equator than through his poles. 



60 bouvier's familiar astronomy. 

Q. Is this diflFerence of length between the polar and equatorial diameters 
of Jupiter perceptible to the eye ? 

A. It is perceptible when the planet is viewed through a good 
telescope. 

Q. What OTHER APPEARANCES does the telescope reveal on the disc of Jupiter? 

A. The telescope shows the disc of Jupiter crossed transversely 
bj belts of comparative shade. 

Fig. 53. 




The disc of Jupiter is an exceedingly interesting object, when viewed with the aid 
of a telescope, on account of its size and subdued lustre. It has the appearance of 
being surrounded by an envelope of reflective clouds drifted into transverse streaks. 
The long dark belts are portions of the body of the planet, seen through transparent 
parts of the atmosphere, where the clouds have separated. The belts frequently change 
their form, indicating changes in the atmospheric conditions of the planet. These 
belts were discovered by G. Fontana, an Italian astronomer, in the latter half of the 
seventeenth century. 

In May, 1850, Professor Schumacher, of Altona, observed four or five white spots on 
one of Jupiter's belts. He says — **The white spots are most remarkable. They are all 
perfectly round, distinct, and bright. The largest spot is as distinct and well-defined as 
the disc of a satellite appears in a nine-foot reflector. They are striking phenomena. 
The spots keep their relative positions as they are carried along by Jupiter's rotation, 
and there are no other similar spots on his disc." — Ast. Journal, vol. i. p. 77. 
Q. Does the telescope reveal any other appearances about Jupiter ? 

A. Yes ; by means of a telescope of moderate power attendant 
moons or satellites may be seen. 

Jupiter's satellites are to him what our Moon is to the Earth. 
Q. How MANY moons has Jupiter ? 
A. Jupiter has/<9ur moons. 

Q. Is the axis of Jupiter inclined to the plane of his orbit ? 
A. The axis of Jupiter is nearly perpendicular to the plane 
of his orbit, being inclined to it about 3° 5'. 

The Sun is always within 3° 5' of Jupiter's equator, consequently his torrid zone is 
only 6° 10' in width, and his polar circles are only 3° 5' from his poles ; so that his 
torrid and frigid zones are extremely narrow, while his temperate zones occupy the 
greater portion of his sphere. 

As the Sun is always vertical near Jupiter's equator, his days and nights are nearly 
always of the same length, except at his poles, which have alternately six years day 
and six years night. 

Q. What is the quantity of light and heat which Jupiter receives fronx the 
Sun as compared with that received by our Earth ? 

A. Jupiter receives only one twenty -seventh as much light and 
heat as we do on the Earth. 



EXTERIOR PLANETS. 61 

Q. Explain this. 

A. Jupiter is four hundred and ninety-four millions of miles 
from the Sun, and the Earth is ninety-five millions of miles from 
that great luminary. It will be found that the distance of Jupi- 
ter, divided by the Earth's distance, will be equal to 5*2, which 
is the number of times Jupiter's distance from the Sun is greater 
than that of the Earth. Therefore, the difference of heat re- 
ceived at the planet Jupiter must equal the square of 5*2, or a 
little more than twenty-seven times less than the quantity re- 
ceived at the Earth. 

But the atmosphere of Jupiter may be of such a nature, that although his inhabitants 
receive less solar heat than we do, yet the temperature of the planet may be as high 



SECTION IV. 
Saturn. I2 

"Last outmost Saturn "walks his frontier round. 
The boundary of ■worlds, ^witb. bis pale moons 
Faint glimmering through, tbe gloom which night has thro-wn 
Deep dyed and dead o'er this chill globe forlorn ; 
An endless desert, "where extreme of cold 
Eternal sits, as in his native seat. 
On \vintry hills of never-thawing ice. 
Such Saturn's earth ; and even here the sight 
Amid these doleful scenes new matter finds 
Of -wonder and delight ; a mighty binq." 

Q. What is the name of the planet next beyond Jupiter in our system ? 
A. Saturn. 

Q. How FAR is Saturn from the Sun ? 

A. About nine hundred millions of miles. 

Q. What is the real size of Saturn ? 

A. Saturn's sphere is about seventy-nine thousand miles in 
diameter, which is about ten times greater than the diameter of 
the Earth. 

Q. Does Saturn rotate on its axis as it revolves round the Sun ? 

A. Yes ; the sphere of Saturn makes one revolution on its 
axis every ten hours and a half. 

Saturn's day is therefore about half an hour longer than that of Jupiter. 

Q. How long is Saturn's year? that is, how long is he 'in performing his 
JOURNEY round the Sun ? 

A. Saturn's year is equal to twenty-nine and a half of our 
years. 

Q. What appearance does Saturn present when viewed through a telescope ? 

A. It is encompassed by a broad ring, and its disc is diversi- 
fied by belts similar to those on the planet Jupiter. 



62 



bouvier's familiar astronomy. 

Fig. 54. 




The belts or stripes of Saturn are variable in their forms. There is a stripe round 
the equatorial region which is of a gray colour, and more uniform in its appearance 
than the rest. The belts do not extend as far as the poles, which being more luminous 
than the other parts of the planet when they have been turned away from the Sun 
during their long winter of fourteen years, led Sir W. Herschel to suppose that th.s 
brilliancy was caused by snow at the poles of the planet. 

Q. Does the telescope reveal any moons belonging to Saturn ? 

A. Yes ; it shows Saturn with eight attendant moons. 

Q. Is the axis of Saturn inclined to the plane of his orbit ? 
A. It is ; the axis of Saturn is inclined about thirty degrees 
to the plane of its orbit. 

Q. What is the consequence of this inclination of his axis ? 

A. A change of seasons. 

Q. Are his seasons similar to ours ? 

A. They are somewhat similar, only o^ greater duration. 

Q. What is the duration of each of his four seasons ? 

A. His spring, summer, autumn, and winter must each con- 
tinue for the space of rather more than seven years. 

Q. Are his polar regions,, like those of the Earth, alternately deprived of 
the LIGHT of the Sun? 

A. They are. 

Q. How long are they in darkness ? 

A. They have alternately fourteen years and three-quarters 
of sunlight, a^ndi fourteen years and three-quarters of darkness. 



EXTERIOR PLANETS. 63 



SECTION V. 

Uranus. i§. 

" God of the rolling orbs above ! 

Thy name is -written clearly bright 
In the -warm day's unvarying blaze, 

Or evening's golden shower of light ; 
For every fire that fronts the Sun, 

And every spark that walli^ alone 
Around the utmost verge of heaven, 

Was kindled at thy burning throne." — Feabody, 

Q. What is the next planet beyond the orbit of Saturn ? 

A. It is called Uranus, and sometimes Herschel, in honor of 
its discoverer. 

Q. How FAR is Uranus from the Sun ? 

A. Uranus is about one tJiousand eight hundred millions of 
miles from the Sun. 

Q. What is the size of Uranus? 

A. Its diameter is thirty-jive thousand miles. 

Q. What is the length of a year on the planet Uranus ? 

A. It revolves in its orbit round the Sun in about eighty- 
four of our years ; consequently, his year is equal to eighty- 
four of ours. 

Q. Who DISCOVERED the planet Uranus ? 

A, It was discovered by Sir William Herschel in the year 
1781, and it is still known by the name of its discoverer. 

Q. Has Uranus any attendant moons ? 

A. Six satellites have been discovered revolving about Uranus, 
but as they are so remote, they cannot be distinctly seen. 

Q. Does Uranus revolve upon its axis ? 

A. It is supposed to revolve on its axis once in about nine 
hours and a half. 

Very little is known concerning this remote planet. The time of its revolution on 
its axis is not yet fully established, nor the inclination of the axis to the plane of its 
orbit; therefore nothing has been ascertained with regard to its seasons. 



SECTION VI. 

Q. Is Uranus the most remote of all the known planets ? 
A. It was deemed so till the year 1846, when a 7iew planet 
was detected beyond the orbit of Uranus. 



64 bouvier's familiar astronomy. 

^. What name has been assigned to this yet more distant planet? 

A. It is called Neptune, or sometimes Le Verrier, in honour 
of its discoverer. 

"A brighter day, a bluer ether, spreads 
Its lucid depth.s above our favoured heads ; 
And purged from mists that veiled our earthly slcies. 
Shine worlds and stars unseen by mortal eyes." — Virgil. 

Q. How FAR is Neptune from the Sun? 

A. Neptune is two thousand eight hundred millions of miles 
from the Sun, or about thirty times farther from that luminary 
than our Earth. 

Q. How LONG is Neptune in performing his journey round the Sun ? 

A. Neptune performs one revolution in his orbit round the 
Sun once in one hundred and sixty four years. 

Q. How LONG, then, is Neptune's year ? 

A. His year is equal to one hundred and sixty four of our 
years, 

Q. What is the size of the planet Neptune ? 

A. It is forty-one thousand five hundred miles in diameter. 

Q. Who FIRST SAW the planet Neptune? 

A. It was first recognised as a planet by Dr. Gf-alle of Berlin ; 
and though it had been frequently seen before, it was always 
supposed to be a fixed star. 

It was observed by astronomers that there were certain inequalities in the move- 
ments of Uranus, which could be accounted for on no other principle than that of an- 
other planet far beyond it. After carefully noting these irregularities, M. Le Verrier 
demonstrated that there must be a planet revolving beyond the orbit of Uranus, and 
even designated in what part of the heavens it might be found ; which calculation 
proved correct. {See Note 16.) 

Q. Is there any variety of seasons on the planet Neptune ? 

A. It is not known whether his axis is inclined to the plane 
of his orbit ; therefore nothing can be ascertained with regard 
to his seasons. 

Q. How long does Neptune require to perform one rotation on his axis ? 

A. Owing to the immense distance of Neptune, his period of 
rotation is unknown to us ; consequently, we have no idea of the 
length of his day. 

Q. Has Neptune any moons ? 

A. One moon has been discovered revolving round the planet, 
and more powerful telescopes may reveal others. 

This moon was discovered by Mr. Lassell in 1846. He believes that he also detected 
a second and smaller satellite in 1850. 

Q. Are there any points of general resemblance in the planets Jupiter, 
Saturn, Uranus, and Neptune ? 

A, Yes ; they are much larger than the other planets of 
our system. The three former are known to be composed 



THE SATELLITES. 65 

of matter much less dense, and to rotate on their axes with 
much greater rapidity, than the planets Mars, Earth, Venus, 
or Mercury. 

Jupiter is eleven times, Saturn ten times, Uranus five times, and Neptune nearly six 
times, the diameter of our earth. 

These four bodies revolve in space at such distances from the Sun, that if it were 
possible to start thence for each, in succession, and to travel at the uniform railway 
speed of thirty-three miles per hour, the traveller would reach 

Jupiter in 1712 years. 

Saturn '' 3113 " 

Uranus " 6226 « 

Neptune " 9685 " 

If, therefore, a person had commenced his journey at the period of the Christian era, he 
would now have to travel nearly thirteen hundred years before he would arrive at the 
planet Saturn, more than four thousand three hundred years before he would reach 
Uranus, and no less than seven thousand eight hundred years before he could reach the 
orbit of Neptune ! And yet the light which comes to us from those remote confines of 
the solar system, first issued from the Sun, and is then reflected from the surface of the 
planet. When the telescope is turned towards that shining point which the astronomer 
has dignified by the name of Neptune, the observer's eye sees the object by means of 
light that issued from the Sun eight hours before, and which since then has passed 
nearly twice through that vast space which railway speed would require almost a cen- 
tury of centuries to accomplish ! ! 



CHAPTER TI. 

S^e Satellite. 



Q. What are satellites ? 

A, They are moons, which serve to enlighten other planets 
called their primaries, as our Moon enlightens the Earth. 

Our Moon is a satellite, of which our Earth is a primary. 
Q. Are the moons or satellites known by any other name ? 

A. They are also called secondary planets. 

Q. Do the satellites revolve round their primaries as a centre ? 

A, They do. 

Q. What is the form of the orbits of the satellites ? 

A. They are usually elliptical, though the orbits of the satel- 
lites of Uranus are supposed to be very nearly circular. 

Q. Have they any other motion besides their revolution round their 
primaries ? 

A. Yes. They accompany their primaries in their journey 
round the Sun, and have also a motion of rotation on their 
axes. 



66 bouvier's familiar astronomy. 

Q. How MANY satellites belong to the solar system ? 

A, The number of satellites which have yet been discovered 
is eighteen. 

Earth has one. 

Jupiter four. 

Saturn eight. 

Uranus four. 

Neptune one. 

Q. Are the satellites visible to the naked eye ? 

A. None of the satellites, except our Moon, can be seen with- 
out the aid of a telescope. 

Q. Are the satellites subject to the same laws of gravitation which govern 
the primary planets ? 

A. The satellites are subject to the same laws of gravitation 
as the primary planets. 

Q. From whence do they derive their light and heat ? 

A. From the Sun. 

Q. Are the satellites as large as their primaries ? 

A, No. The satellites are always smaller than their primaries. 

The Moon is smaller than the Earth, which is its primary. 



SECTION I. 

€\t Poon. 
Q. What is the Moon? 
A. The Moon is a satellite, of which our Earth is the primary. 

Q. Has the Earth more than one satellite ? 

A. No ; our earth has but one satellite or attendant moon. 

Q. Is the Moon composed of solid substance ? 

A. Yes ; the Moon is a solid globe of matter, receiving and re- 
flecting the light of the Sun. 

"As "w-hen the Moon, refulgent lanap of night. 
O'er heaven's clear azure spreads her sacred light. 
When not a breath disturbs the deep serene. 
And not a cloud o'ercasts the solemn scene; 
Around her throne the vivid planets roll. 
And stars unnumbered gild the glowing pole; 
O'er the dark trees a yellower verdure shed. 
And tip with silver every- mountain's head ; 
Then shine the vales, the rocks in prospect lise— 
A flood of glory bursts from all the skies." 

Q. How is it known that the Moon is composed of solid matter ? 

A. If we examine the Moon, even without the aid of a tele- 
scope, spots of light and shade will be seen upon its disc, evi- 
dently proving the existence of permanent varieties of surface. 



THE SATELLITES, 



67 



Q. Do these delineations of light and shade appear through the telescope ? 

A» Yes; the telescope shows certain fixed inequalities m the 
Moon's surface, such as only belong to a body composed of solid 
materials. 

The surface of a fluid body must either assume the appearance of a smooth, unbroken 
plain, or else it must be constantly changing its aspect with a wavelike motion ; but 
the Moon has certain fixed spots on its surface, which are a proof that it is not com- 
posed of fluid matter. 

Q. Does the dense matter of which the Moon is composed ixfluencb the Earth ? 

A, The force of its attraction extends to the Earth. 

Q. In what way is its attraction shown on the Earth ? 

A. The tides rise chiefly in obedience to the attraction of the 
Moon's substance. 

Q. What indicates the force of the Moon's attraction ? 

A. The height to which the tides are raised show the power 
of the Moon's attraction, or, in other words, its density. 

Q. Does the Moon shine by reflected sunlight? 

A. It does ; for if, like the Sun, she were a self-luminous body, 
she would, like him, always shine with aifull orb; but when only 
a portion of the Moon's enlightened hemisphere is visible to us, 
the remainder is invisible, because it is not enlightened by the 
Sun's rays. 

Q. At the time of new moon only a small crescent of its illuminated hemi- 
sphere is visible, yet the outline of the full orb can be distinguished: explain this. 

A, The crescent is illuminated by the Sun ; but the remaining 
part of the Moon's disc is enlightened by the sunlight reflected 
from our Earth. 

Q. What is this reflected sunlight to the inhabitants of the Moon ? 

A. It is earth-light, or their moonlight ; for our Earth gives 
light to the Moon, as the Moon does to us. 

Q. What is the distance of the Moon from the Earth ? 

A. The Moon is about two hundred and forty thousand miles 
from the Earth. 

Q. How can the Moon's exact distance be ascertained ? 

A. By observing the different positions it seems to occupy in 
the heavens, when viewed by two observers placed widely asun- 
der on the Earth's surface. 

Q. What is the size of the Moon ? 

A. The Moon is about two thousand one hundred and sixty 
miles in diameter. 

Q. How LARGE would the Earth appear if viewed from the Moon ? 

A. The Earth would appear thirteen times larger than our Moon 
does to us. [See Note 17.) 

Q. What is the difference between the bulk of the Moon and that of the Earth ' 

A. The bulk of the Earth is ne'dr\j fifty times greater than 
that of the Moon. 



68 BOUVIER S FAMILIAR ASTRONOMY. 

To find the bulk or solid contents of a sphere, multiply the cube of its diameter by 
the decimal -5236. Thus, the solid contents of the globe of the Earth is 264,082,024,729 
miles, supposing its diameter to be 7960 miles. The solid contents of the Moon, sup- 
posing her diameter to be 2160 miles, equals 5,276,681,625 miles; whence it follows, 
that the bull:, or solid contents of the Earth, is nearly fifty times greater than that of 
the Moon. 

Q. Is the Moon's substance as dense as the Earth's substance ? 

A. No ; the Moon's substance is rather more than one-half less 
dense than the Earth's substance. 

Q. Of what DENSITY is the Earth ? 

A. The Earth is as dense or heavy as it would be if composed 
entirely of metallic iron ore, a substance which is more than five 
times heavier than water. 

Q. Of what DENSITY is the Moon ? 

A. The Moon's sphere is as heavy as it would be if composed 
entirely of diamond ; a substance which is more than three times 
heavier than water. 

Q. Platinum is twice as heavy as silver : if two bullets were made of equal 
size, one of platinum and the other of silver, how much greater would be the 
mass of the platinum bullet than of the silver one ? 

A. The mass of the platinum bullet would be equal to two sil- 
ver bullets; that is, its mass would be tivice as great as the silver 
bullet, notwithstanding the equality of their bulks. 

Q. The Moon has but a fiftieth part of the bulk of the Earth, and its sub- 
stance is much lighter ; how much, then, is the Moon's mass less than the Earth's 



A. The lunar sphere has, in itself, only an eightieth part of 

the Earth's mass. 

Q. How can the density and absolute 3iass of the Moon's substance be 
ascertained ? 

A. By observing the power the Moon exercises in attracting 
other bodies. 

Q. In what proportion is the attractive power of a body ? 

A. The attractive power of a body is always in proportion to 
its size and density. 

Q. What is the form of the Moon ? 

A. It is spherical. 

Q. How is it known that the Moon is spherical ? 

A, That part which is visible always presents the same appear- 
ance to us which we know a sphere would do, if suspended in the 
sunlight. 

Q. How many motions has the Moon ? 

A. Three; one on her own axis, the second in her orbit round 
the Earth, and the third as an attendant on the Earth round 
the Sun. 

Q. How LONG is the Moon in making one revolution on her axis? 

A. The Moon revolves once on her axis in about tiventy -seven 
days and a quarter. 



THE SATELLITES. 69 

Q. How long is the Sun above the horizon on the Moon ? 

A, As the Moon revolves on her axis once in about twenty- 
seven days, her term of daylight must be the half of twenty-seven 
days, or nearly two weeks, and her night must be of the same 
length. 

Q. Does the Moon revolve round the Earth ? 

A. Yes ; the Moon is a constant companion of the Earth round 
the Sun. 

"Ho-w calraly gliding through the dark blue sky 
The raidnight Moon ascends!" — Scut hey. 

Q. How MANY TIMES does the Moon revolve on its axis while it is accompany- 
ing the Earth in a revolution once round the Sun ? 

A. The Moon revolves once upon its axis and moves once round 
the Earth in almost exactly the same length of time; therefore the 
Moon must revolve on its axis about thirteen times in one year. 

Q. Can any inequalities or spots be seen on the face of the Moon ? 

A. Yes ; the Moon's disc may be seen to be variegated by 
darker and lighter spots, even by the unaided eye. 

Q. Can the motion of the Moon on its axis be detected by the movement of the 
spots on its surface ? 

A. No ; for the same side of the Moon is constantly turned 
toward the Earth. 

Q. Do the spots hold the same relative position with regard to the surface 
of the Moon ? 

A. They do; and also hold the same relative position with 
regard to each other. 

Q. As the Moon revolves on its axis, why are not these spots turned away 
from the Earth ? 

A. This is the inevitable result of the Moon revolving once 
round the Earth, and once upon its axis in nearly the same length 
of time. 

Place a terrestrial globe, or any other body, upon the centre of a round table, and 
stand looking at it with your back to the fire ; then move round the table, keeping your 
face constantly towards the globe on the middle of the table, and you will find that when 
you have been once round the table, you will also have made your body turn once round 
upon itself; for your back is again towards the fire. If you had pinned one end of a 
long thread to your back before you started to go round the table, and had tied its other 
end to the poker in the chimney corner, you would find that you had wound the thread 
once round your person when you had completed your revolution round the table. In 
the same manner does the Moon revolve round the Earth, keeping the same side always 
turned towards us. Thus, it must revolve once upon its axis when it has completed one 
sidereal revolution round the Earth. 

Q. Is one side of the Moon's sphere, then, never turned towards the Earth? 

A. Yes ; there is one lunar hemisphere which is never visible to 
terrestrial eyes. 

Consequently, the inhabitants, if any, of that half the Moon which is invisible to us, can 
never see the Earth, unless they travel round to the hemisphere which is turned towards us. 
Q. Do the times which the Moon requires to revolve upon its axis, and to re- 
volve round the Earth, correspond exactly ? 

A. No ; they do not correspond precisely, though very nearly. 



'0 



BOUVIER S FAMILIAR ASTRONOMY. 



Q. What is the effect of this slight variation in the times of the Moon's revo- 
lution round the Earth and round its own axis? 

A. The inhabitants of the Earth are enabled to see, sometimes 

a little farther round one side of the Moon, and sometimes a little 

farther round the other side. 

Q. What is the vibration, or showing a little on one side, and then a little on 
the other side, called ? 

A. It is called the Moon's lihration in longitude, 

Fio:. 55. 





In fig. 55, E represents the Earth, and A B C D the Moon in her elliptical orbit round 
the Earth : from D through A to B the Moon's orbitual motion is slower than her revo- 
lution on her axis, by which means we are enabled to see more of her eastern limb ; but 
from B through C to D her orbitual motion is faster than her revolution on her axis, 
which enables us to see more of her icestcm limb. 

Q. Is there any other vibration perceptible in the Moon? 
A. Yes ; sometimes a little more of the noHli pole, and then a 
little more of the south pole is brought into view. 

Q. What is the showing sometimes a little more of one pole, and then a little 
more of the other pole, called ? 

A. It is called the Moon's lihration in latitude. 



\ 




Suppose in the figure, E to be the Earth, and M M the Moon moving in U. its ellipticiil 
orbit; the Moon's axis of rotation is placed perpendicularly to the plane of its orbitual 



THE SATELLITES. 



Tl 



movement, as the lines P P in the figure are perpendicular to the curved line 0. 
Now, the reason that sometimes one pole, and sometimes the other, is more brought 
into view, is, that the Moon's axis is so very little inclined to the plane of its orbit. 
This vibration is called the Moon's libration in latitude. 

Q. Is the Moon's axis inclined with regard to the direction in which it moves 
round the Earth ? 

A. Yes ; though the Moon's axis is very nearly perpendicular 
to the plane of its orbit round the Earth, it has the small inclina- 
tion of 1° 30' 10''. 

Q. Does the Moon always present itself to the inhabitants of the Earth under 
the form of an enlightened hemisphere ? 

A. No ; sometimes only di portion of its enlightened hemisphere 
is visible to us, as in the following figure. 

Fig. 57. 




This figure represents a telescopic view of the Moon, taken by the daguerreotype pro- 
cess, at the Cambridge Observatory, Massachusetts. 

Q. When is a.ll of the enlightened half of the Moon visible to us? 

A. At the period oifull moon, 

-"Tin the Moon, 

Risicg in clouded majesty, at length, 

Appc'.cent queen, unveiled her peerless light. 

And o'er the night her silver mantle threw." — Milton. 

Q. Wliy is all the enlightened half of the Moon's sphere visible at the time of 
full moon? 

A. Because the Earth's inhabitants see the Moon's sphere in 
the same direction from whence the Sun's rays pass to illuminate it. 



72 



BOUVIER S FAMILIAR ASTRONOMY. 



Fi-. 5S. 




In the above figure let E represent the Earth, and ABC D. Ac. the Moon in differ- 
ent parts of her orbit It is plain that when the Moon is at A. in that part of her orbit 
between the Earth and the Sun. her enlightened hemisphere is turned away from us,. 
consequently she is invisible. As she advances in her orbit from A to C, she first ap- 
pears a slender crescent, which increases in size uU at C one-half the disc towards us is 
enlightened; at this period she is said to be in her jirst quarter. From C to F she 
shows still more and more of her enlightened hemisphere, till at F, being directly oppo- 
site to the Sun, her disc appears perfectly round. She is then fuU. From F, her en- 
lightened half is only partly visible, and on her arrival at H only half of her illu- 
minated hemisphere can be seen. In this position she is said to be in her iast quarter. 
From H to A she appears to diminish in size till she becomes a narrow crescent, and a: 
A is finally lost in the Sun's beams, — her dark side being entirely turned towards us. 

In the figure, m n represents the half of the Moon which is turned towards the Earth, 
and b d a. line joining the centres of the Earth and Moon. "When the Moon is at A, its 
dark side is turned towards us as at a : at B only part of her illuminated disc can be seen 
as at b: at C she has half her enlightened side towards us. and then appears a half moou 
as at c: at F she is directly opposite the Sun, and presents a full orb as at/. At B and 
I she is a crescent: at D and G she appears gibbous. When at C she is in her jirat 
quarter, and at H in her last quarter. 

When at A the Moon is in conjunction / that is, she appears in the same quarter of 
the heavens with the Sun : and when at F she is in opposition ; that is, in that part of 
the heavens opposite to the Sun. The two positions of the conjunction and opposition 
are called syzigiea, and those of the first and last quarters quadratures. The position at 
B is called the first octant, D the second octant, Gr the third octant, and I the fourth 
octant, 

Q. When can xo part of the enlightened half of the Moon's sphere be seen? 
A. At the period of 7iew moon., or when at A, [iig. dS.) 
Q. Why can xo portion cf the enlightened half of the Moon be seen at the 
period of new moon ? 

A. Because to the Earth's inhabitants the Moon's sphere is in 



THE SATELLITES. 



73 



the same direction as the Sun, the dark half being turned to- 
wards us, and the Sun's light falls upon the other, for which 
reason it is invisible to us, as at A, {fig- 58.) 

If a candle be placed upon a table before an observer, and a small round ball be held 
between its flame and the eye, the eye only rests upon the half of the ball which is not 
illuminated by the candle, and the half enlightened by the direct rays of the candle is 
invisible, because turned from us. 

Q. What APPEARANCE does the Moon assume "when its enlightened half is 
neither altogether visible nor entirely concealed ? 

A, It assumes the various forms from a narrow crescent to 
almost a full circle, being a little flattened on one side. 

"The Queen of Nighc 
Shines fair witii all her virgin stars about her." — Ottvay. 

Q. The Moon's hemisphere sometimes presents the appearance of almost a full 
round disc : what is this form called ? 

A, The Moon is then said to be gibbous. 

Q. By what name are the varying appearances of the Moon known ? 

A. They are called the phases of the Moon. 

Q. How are the varying appearances of the lunar phases produced ? 

A. By the gradual and successive appearance and disappear- 
ance of the enlightened hemisphere of the Moon. 



" By thy command the Moon, as daylight 
Lifts her broad circle in the deepening shades ; 
Arrayed in glory, and enthroned in light, 
She breaks the solemn terrors of the night ; 
Sweetly inconstant in her vaiying flame. 
She changes still, another, yet the same ! 
Now in decrease, by slow degrees she shrouds 
Her fading lustre in a vail of clouds ; 
Now of increase, her gathering beams display 
A blaze of light, and give a paler day. 
Ten thousand stars adorn her glittering train. 
Fall when she falls, and rise with her again ; 
And o'er the deserts of the sky unfold 
Their burning spangles of sidereal gold ; 
Through the wide heavens she moves serenely bright. 
Queen of the gay attendants of the night; 
Orb above orb in sweet confusion lies, 
/ind with a bright disorder paints the skies." — Bboomh. 

Q. How is it that different proportions of the enlightened half of the Moon 
are visible to the inhabitants of the Earth at different times ? 

A. Because the Moon is constantly changing her position with 
regard to the Earth and Sun. 

Q. What is the motion of the Moon with regard to the Earth and Sun ? 
A. It moves round the Earth in an elliptical orbit, and also 
accompanies the Earth in her yearly revolution round the Sun. 



74 



BOUVIER S FAMILIAR ASTRONOMY. 



Q. In what part of the Moon's elliptical orbit is the Earth placed ? 

A. The Earth is placed in one of the foci of the ellipse, just aa 
the Sun occupies one of the foci of the Earth's elliptical orbit. 




In fig. 59, AB C represents the orbit of the Earth, and M that of the Moon. The 
Moon M revolves round the Earth E, and also accompanies the Earth in her orbit 
round the Sun S. 

Q. With what velocity does the Moon move in her orbit ? 

A, She moves in her orbit round the Earth at the rate of 
about two thousand three hundred iniles per hour. 

Q. What is the apparent angular motion of the Moon in a day ? 
A. The Mopn appears to move through rather more than 
thirteen degrees of the circumference of the heavens in one day. 

Q. If, then, the Sun be in conjunction with the Moon on any given day, how 
MANY DEGREES of angular measurement will she be from him on the following 
day? 

A. The Sun only appears to move about one degree per day, 
while the Moon moves more than thirteen ; therefore they will be 
more than twelve degrees asunder on the following day. 

Q. The Moon's orbit is an ellipse ; but is it a regular curve ? 

A. No ; the Moon's orbit is an irregular curve, always concave 
towards the Sun, 

Fig. 60. 




THE SATELLITES. 



75 



The plain line A E C E F represents the orbit of the Earth, and the dotted one that 
of the Moon. At A the Moon crosses a point of the Earth's orbit which the Earth haa 
already passed. At the expiration of about one-fourth of a lunation she arrives at B, 
at which time the Earth is between the Moon and the Sun, — consequently it is full 
moon ,• pursuing her course, she is now in advance of the Earth, and crosses her orbit 
at C ; from C she continues her course, till at D she is between the Earth and the Sun, — 
consequently it is new moon; from D she approaches nearer and nearer to the orbit of 
the Earth, till at F she again crosses it, 240,000 miles behind the Earth. This com- 
pletes one lunation or revolution of the Moon round the Earth. 

That the orbit of the Moon is concave towards the Sun, may be explained thus : Let 
A G C be a chord of the arc A B C, a part of the Moon's orbit, and C H F a chord of the 
arc CDF, another portion of the Moon's orbit : it will be seen that the dotted line 
representing the Moon's orbit is always concave towards the Sun. Thus, AGO is a 
chord of the arc A E C, which is a part of the Earth's orbit, and the dotted line ABC, 
representing a portion of the Moon's orbit, is evidently concave towards the sun S ; so 
C H F is a chord of the arc CDF, another portion of the Moon's orbit, and this chord 
being a straight line, it follows that the arc CDF must be concave towards the Sun S. 
Now, as this figure represents one lunation, it will be seen that the Moon has really 
made one complete revolution round the Earth as she moved in her orbit from A to F. 

Q. How is the Moon upheld in her place in space ? 

A. By the combined influence of the centrifugal and centri- 
petal forces ; that is, the projectile force of the Moon, and the 
attraction of the Earth ; or in other words, by the influence of 
motion and attraction, 

Q. How LONG does it take the Moon to complete one of her elliptical journeys 
round the Earth ; that is, from one star to the same star again ? 

A. Twenty-seven days and three tenths of another day. 

Q. What is that period called in which the Moon completes a revolution 
around the Earth ; that is, from one star to the same star again ? 

A. It is called a sidereal revolution, or period of the Moon. 

Q. How is it known when the Moon has completed one of its elliptical journeys 
round the Earth ? 

A. It is observed that the Moon has returned to the same re- 
lative position with regard to the stars. 

Q. When the Moon is seen in conjunction with a star, does it seem to touch 
the same star again when it has completed one revolution ? 

A. No ; when it has completed its revolution, it is either above 

or helow the place it last occupied when in that part of the 

heavens. 

Q. How is it known that the Moon does not move round the Earth in a per- 
fectly CIRCULAR path ? 

A. The Moon sometimes measures more than at other times, 
and therefore must be nearer to the Earth sometimes than at 
others. 

The Moon's disc sometimes measures 4 minutes 10 seconds more than at others. She 
seems, also, to move faster with regard to the fixed stars than at other times. Her 
motion is greatest when nearest to the Earth; for then her momentum is increased, to 
resist the power of the Earth's attraction. 

Q. Does the Moon complete a succession of her phases in the same time in 
•which she completes one revolution about the Earth ? 

A. No ; a succession of phases occupies more than tivo days 
longer than one revolution about the Earth. 



76 



BOUVIER S FAMILIAR ASTRONOMY. 



Q. What is the exact time which the Moon requires to complete a succession 
of phases ? 

A. About twenty-nine days and a half. 

Q. What is this period called in which the Moon completes one series of phases ? 

A. A synodic revolution of the Moon, or lunar month. 

" Then marked Astronomers with keener eyes 
The Moon's refulgent journey- through the skies." — Da3wi>j. 

Q. Why does the Moon require longer to complete a succession of its phases, 
than to complete a revolution round the Earth ? 

A. Because when the Moon has gone once round the Earth, 
the Earth has advanced in her orbit in the same general direc- 
tion. Thus, the Moon must move on a certain time longer before 
it is again between the Earth and the Sun, to mark the com- 
mencement of a new lunation. 

Fig. 61. 




Let S represent the Sun, and m the Moon between the Earth E 1 and the Sun. Let 
St St represent a particular star. Now, when the Moon has moved once round the 
Earth, which is known by its coming once more between that particular star St and the 
Earth, the Earth will have advanced in its orbit from the position 1 to the position 2 ; 
but the Moon will not then be again between the Sun and the Earth. It must move on 
to n before it is so, and the additional time it requires to come again between the Sun 
and the Earth is the difference between the periods in which one succession of the lunar 
phases and one lunar revolution round the Earth are completed.* 
Q. Is the Moon's axis ixclixed to its own orbit ? 

A. The Moon's axis is very nearly upright or perpendicular to 
the plane of its orbit, being inclined to it only 1° 32' 9'\ 

Q. How much is the Moon's orbit ikclixed to the plane of the ecliptic ? or, in 
other words, what axgle does the Moon's orbit make with the orbit of the Earth ? 

A. The Moon's orbit is inclined, or makes an angle of about 
five degrees with the orbit of the Earth. 



* The two parallel lines St St are drawn to represent the direction of ove and tht 
same star, because the stars are so immensely distant from us, that the lines leading to 
them from the Earth in different parts of its orbit can never be found to have any con- 
vergence towards each other; they are so nearly parallel, that practically ihey are 
quite 60. 



THE SATELLITES. 77 

Q. The path or orbit of the Moon crosses the Earth's orbit in two points : 
nrhat are those two points called ? 

A. Thej are called the Moon's nodes. 

Let the edge of a circular table represent the orbit of tlie Earth round the Sun ; the 
surface of the table would be the plane of the orbit. Now, the Moon revolves round the 
Earth in an orbit inclined 6° to the Earth's orbit; so that one-half of the Moon's orbit 
would lie 5° below the surface of the table, and the other lialf 5° above it. The points 
of intersection with the edge of the table would represent the Moon's nodes. 
Q. Has the Moon the same variety of seasons with the Earth ? 

A. She has not. As her axis is nearly perpendicular to the 
plane of her orbit, she cannot have any material change of seasons. 

The half of the Moon towards us has the advantage of reflected light from the Earth ; 
for in the absence of the Sun it is illuminated by our Earth, wbich would appear thir- 
teen times larger than the Moon does to us. The inhabitants have two weeks of sun- 
shine, and two weeks of moonlight. But the inhabitants on the other side of the Moon 
never see our Earth; consequently, they have no moonlight, but two weeks of sunlight 
and two weeks of darkness. 

Q. What would be the appearance of the nocturnal sky to the inhabitants of 
that hemisphere of the Moon which is always turned away from us ? 

A. The heavens would have the same appearance that our own 
sky does on moonless nights, except that as the Moon revolves so 
slowly on her axis, the stars would be fourteen of our days from 
their rising to their setting. 

Q. What would be the appearance of the nocturnal sky to the inhabitants of 
that hemisphere of the Moon which is always turned towards us ? 

A. They would see our Earth, which would be to them a moon^ 
presenting a disc thirteen times larger than the Moon's disc, and 
passing through all the phases from a crescent to a full moon once 
a month. 

Q. Would the Earth seem to them to rise and set, as the Moon does to us ? 

A. No ; the Earth would seem almost fixed in the lunar sky. 

If a spectator were placed on the surface of the Moon, at the centre of the hemisphere 
towards us, he would see our Earth revolving on her axis once in twenty-four hours, 
and presenting all the phases of the Moon once in a month. Our Earth would be varie- 
gated by spots corresponding to the clouds, and belted, owing to the trade-winds. At 
the time of an eclipse of the Sun to them, the atmosphere of our Earth would appear 
like a halo, or ring of a ruddy color fading into pale blue. 

Q. Does the Moon reflect on the Earth any portion of the solar heat which 
she receives ? 

A. It is generally believed that there is no perceptible heat in 
moonlight. {See Note 18.) 

Q. Why does the Moon move round the Earth ? 

A. Because the Earth's substance attracts the Moon's substance. 

Q. Then, why does not the Moon fall to the Earth ? 

A. Because as the momentum of the Moon's velocity impels it 
in a different direction to the attractive power of the Earth, it 
consequently obeys neither, but pursues a path between the two. 

Q. Does not the Moon's substance attract the Earth's substance as well as 
suffer attraction by it ? 

A. It does; but as the Moon's mass is so much less than the 
Earth's mass, her attractive power is proportionably less. 



<0 BOUVIER 5 FAMILIAR ASTRONOMY. 

Q. Does the Snn's substance also influence the Moon's substance? 

A. Yes : the Moon is attracted hy the Su7i as well as by the 
Earth? 

Q. How is the attractive lstluence of the Sun over the Moon made manifest ? 

A. By certain perceptible irregularities of movement which the 
Moon suffers. {See Xote 19.) 

Q. Has the Moon an atmosphere ? 

A. It is supposed to be void of atmosphere of sufficient density 
to be worthy of the name of air. 

Q. Are there any clouds surrounding the Moon ? 

A. The moon has 7io clouds, nor any other decisive indications 
of an atmosphere. 

If the Moon has an atmosphere at all like ours, clouds could be perceived floating in 
it : but no such appearances have ever been detected. Besides, the outlines of all the 
inequalities on its surface are sharp and well-defined j which would not be the case if 
there was an atmosphere similar to ours. 

Q. How can the absence of an atmosphere in the Moon be proved ? 

A. Should there be any appreciable amount of vapor suspended 
near the surface of the Moon, the light of very faint stars would 
be diminished or extinguished before occultation, which is not 
the case 

Q. During a total eclipse of the Moon is there any indication of an atmo- 
sphere ? 

A. Very minute stars are seen to pass under the edge of the 
Moon, and undergo sudden extinction; whereas, if the Moon had 
an atmosphere at all like ours, thev would disappear gradually. 
[See Note 20.) 

M. M. Baer and Miiedler, two noted German astronomers, who have made the most 
extensive selenographical researches hitherto attempted, have arrived at the conclusion 
that the Moon is not eiitireb/ icithout an atmosphere, but owing to the smallness of her 
mass she is incapacitated from holding an extensive covering of gas ; and they add, 
" It is possible that this weak envelope may sometimes, through local causes, in some 
measure dim or condense itself." But if any atmosphere exists on our satellite, it must 
be, as La Place says, more attenuated than what is termed a vacuum in an air-pump. 

Q. Does the Moon rise at the same hour every day ? 

A. No; she rises about three-quarters of an hour later each 
day than on the one preceding. 

Q. Why does she rise later and later every succeeding day ? 

A. Because the Moon advances in her orbit about thirteen de- 
grees per day, on which account any place on the Earth must 
make more than one complete rotation before it arrives again in 
the same situation with regard to the Moon ? 

Q. Does the Moon rise about three-quarters of an hour later every day to all 
'=LACES on the Earth ? 

A. The difference of time of the Moon's rising at the equator 
is nearly three-quarters of an hour, or, more properly, fifty 
minutes; but in high latitudes the difference is much less in the 
months of August and September. 



THE SATELLITES. 



79 



Q. Why is there less difference in the times of the Moon's rising in the 
months of August and September than at any other time of the year ? 

A. Because in the months of August and September the Sun 
is in the signs Virgo and Libra, which places the full moon in the 
opposite signs of Pisces and Aries ; and in high latitudes as much 
of the ecliptic rises about Pisces and Aries in two hours as the 
Moon passes through in six days ; therefore, when the full moon 
is in these signs, she differs in the time of her rising but about 
twenty minutes a day, for nearly six days. 

Q. Does the full moon in these months render any essential service to hus- 
bandmen ? 

A. Yes; because in the months of August and September, 
which are the harvest months to the inhabitants of the higher 
latitudes, the husbandmen have the light of the full moon for 
several consecutive nights imynediately after sunset, which enables 
them to gather in their grain. 

Q. Was the phenomenon of the Moon's rising after sunset for several nights 
together, known to the ancients ? 

A. Yes ; it was observed by persons engaged in agriculture at 
a much earlier period than it was noticed by astronomers, and has 
been long known by the name of the Harvest Moon. 

"There is a time -well kno^wn to husbandmen. 
In which the Moon for many nights, in aid 
Of their autumnal labors, cheers the dusk 
With her full lustre, soon as Phoebus hides 
Beneath the horizon his propitious ray ; 
For as the angle of the line -which bounds 
The Moon's career from the equator, flows 
Greater or less, the orb of Cynthia shines 
With less or more of difference in rise ; 
In Abiss least this angle; thence the Moon 
Rises with smallest variance of times 
When in this sign she dwells ; and m.ost protracts 
Her sojourning in our enlightened skies." — Lobtt. 

During the latter part of the harvest months, or when the Moon is in Aries, it is called 
the Hunter's Moon. At the poles, one-half of the year, the ecliptic never sets, and during 
the other half it never rises; consequently, the Sun continues one-half the year above 
the horizon, and the other half below it. The full moon being always opposite to the 
Sun, can never be seen to the inhabitants of the poles during their summer, but in win- 
ter they have the benefit of her light for the space of fourteen days and nights at a 
time, without intermission. The inhabitants of the poles are not deprived of the light 
of the Sun for quite six months, because the atmosphere refracts'^ the Sun's rays, by 
which means he becomes visible about two weeks earlier, and continues to be seen two 
weeks later, than he otherwise would do. The inhabitants of the polar regions have a 
tolerable share of light afforded them by the meteoric phenomenon called the Aurora 
Borealia. 

"By dancing meteors, then, that ceaseless shake 
A waving blaze refracted o'er the heavens, 
And vivid moon's and stars that keener play 



* See Part iii. chap. i. sec. i. div. iii. 



80 BOUVIER S FAMILIAR ASTRONOMY. 

With, double lustre from the glossy waste ; 

Ev'n in the depth of polar night, they find 

A wondrous day; enough to light the chase. 

Or guide their daring steps to Finland fairs." — Thomson. 

Q. Why does the Moon's disc appear mottled ? 

A. Because some parts reflect the Sun's rays, and other por 
tions are in shadow. 

Q. How can these various appearances in the Moon's disc be accounted for ? 

A. The brighter portions are elevations^ which receive the Sun's 
rajs, while the darker portions are cavities^ or comparatively level 
tracts, which reflect less sunlight. 

Q. Can these elevations and cavities be discerned by the naked ete ? 

A. No ; but by means of the telescope we can aid the eye in 
making the investigation. 

By the unaided eye we can observe the lights and shadows on the Moon, but we can- 
not discern them to be inequalities on its surface. 

Q. How can the telescope aid our sight? 

A. It makes any celestial body appear much nearer to the ob- 
server than it really is. 

Q. How can it make the Moon appear neaker to us ? 

A. The object-glass of the telescope collects more rays from 
any visible object than the eye can, and the eye-glasses of the 
telescope, by magnifying the object, spread its image over a 
larger sensitive surface within the eye, and therefore allow the 
details to be seen more accurately and distinctly. 

Q. How NEAR can the Moon be made to appear by the aid of a telescope ? 

A. The Moon being so much nearer to us than any other 
heavenly body, the telescopic power is more conspicuous when 
directed to it. The surface of the Moon can be as distinctly seen 
by a good telescope magnifying 1000 times, as it would be if not 
more than two hundred and fifty miles distant. 

Q. What DISCOVERIES have been made, by means of the telescope, on the surface 
of the Moon ? 

A, Elevations of great dimensions have been discovered on the 
Moon, which cast distinct shadows when the Sun shines obliquely 
on them. Deep cavities, also, are to be seen, unlike any thing 
of the kind on our globe. 

Q. When do terrestrial objects cast the longest shadows ? 

A. At the time of sunrise or sunset ; that is, when the Sun's 
rays fall the most obliquely upon them. 

For this reason, when the Sun shines obliquely, or when he is rising or setting to that 
side of the Moon towards us, the shadows cast by the lunar elevations are the longest 
and most plainly seen by the aid of the telescope. 

Q. To what part of the visible half of the Moon is the Sun rising or setting 
at any given time ? 

A. It is rising or setting on the line which separates the illu- 
minated from the dark part of the Moon's disc. 



THE SATELLITES. 

Fi.o:. 62. 



81 




Suppose, in the above figure, M to be the Moon with its enlightened half turned to- 
wards the Sun S : then it would be sunrise or sunset along the border of the light half 
where it joins to the dark, indicated in the figure by the line of dots ; for that is the part 
of the spherical surface just emerging into the Sun's rays or receding from them j while 
along that part marked a a a it would be midday. 

Q. When the Sun shines vertically over any part of the Earth, are there any 
shadows cast? 

A. No ; there are no shadows cast at those places under a 
vertical sun. 

Fig. 63. 




Fig. 63 is a telescopic representation of the full Moon taken at the Cambridge (Mas- 
eachusetts) Observatory, by the daguerreotype process. 

" Lo ! the "beauteous Moon 
Like a fair stepb-erdess, now comes abroad 
With, the full flock of stars, that roam around 
The azure meads of heaven." — Robert Montgomsry. 
Q. Why cannot long shadows be seen at the time of full moon where the solar 
rays fall obliquely ? 

A. Because the shadows are then cast behind the projecting 
objects as regards the position of the observer, as well as regards 
the position of the Sun. 



82 



BOUVIER S FAMILIAR ASTRONOM^ 



Q. At the time of full moon, in what position is the observer with regard to 
the Sun and Moon ? 

A. The Earth is then between the Sun and Moon ; conse- 
quently, we look upon the Moon in the same direction from 
whence the rays of the Sun proceed. 

Q. When can the shadows cast behind projections from the lunar surface be 
seen most adtaxtageouslt ? 

A. At the first and last quarters of the lunation ; or in other 
words, at the time of new moon and old mooji. 

Q. Why are the shadows most pebceptible at the fiirst and last quarters of 

the lunation ? 

A. Because the observer can then see the full length of the 
shadows cast behind the eminences. 

Fig. 64. (Lr:<AR SHAiwws.) 




Fig. 17 (Lunar Shadows) represents the appearance of a shadow cast behind a pyra- 
midal elevation on the Moon's surface on the eighth day after new moon. This jieak 
is near the upper part of the Moon's disc, and is denominated Pico. 



THE SATELLITES. 



83 



Q. Why are the lunar shadows objects of great interest for terrestrial 
observers ? 

A. Because they enable them to ascertain the height of the ele- 
vations of the Moon's surface, and because they render the forms 
and outlines of those elevations more distinct than they would 
otherwise be. 

Q. How can the height of a lunar mountain be ascertained by observing its 
shadow ? 

A. The length of a shadow is always proportioned to the height 
of the body casting it. 

.Q. Does the inclination at which the light falls on a body aid in determining 
its height? 

A. It does. Whenever light falls on a body at an inclination 
of half a right angle, that is, 45° above the horizon, a shadow is 
cast exactly as long as the body is high. 

Q. When the light falls with a less inclination than 45° above the horizon, 
is the shadow longer or shorter than the object ? 

A. When the light falls with a less inclination than 45° above 
the horizon, the shadow is proportionately lengthened; when it falls 
with a greater inclination, the shadow is proportionately shortened, 
until it arrives at the zenith, when there is no shadow at all. 

Q. To what extent do the lunar elevations rise above the general surface of 
the sphere? 

A. Some of them rise to the height of twenty-four thousand 
feet; an elevation nearly as great as that of the highest peaks of 
the Andes on our globe. 

Q. Are the mountains on the Moon higher in proportion than those on our 
Earth ? 

A. The mountains on the Moon are on a much grander scale 
than those on our Earth. If Chimborazo were as high in propor- 
tion to the Earth's diameter as a mountain in the moon known by 
the name of Newton is to the Moon's diameter, its peak would be 
more than 16 miles high. 

Q. How do the lunar shadows increase the distinctness of the forms of eleva- 
tion on the Moon's surface ? 

A. They make otherwise inconspicuous elevations more distinct 
by means of the dark shadow which borders them. 

Fig. 3 (Lunar Shadows) shows the appearance of the commencement of a chain of 
lunar peaks, known by the name of the Apennines, on the tenth day of the lunation. 
Fig. 4 of the figure is a sketch of the same chain on the eighth day, when long shadows 
are cast behind them. On account of the distinctness with which objects seem to stand 
out when bordered by their dark shadows, they appear to be many times larger when 
accompanied by their shadows than they do when without them. 

Q. Do the lunar elevations wear the same general form as the terrestrial 
mountains ? 

A. With, few exceptions, the form of the lunar mountains dif- 
fers from those on our Earth. 

Q. What, then, is the usual appearance of the lunar mountains ? 

A. In the vast majority of instances they have the form of 



84 bouvier's familiar astronomy. 

circular ridges surrounding cavities^ and these cavities have fre- 
quently isolated peaks rising from their centres. 

Fig. 6 (Lunar Shadows) shows the appearance of one of these circular ridges, which 
may be selected as a type of the whole. The sketch is taken from an object which the 
selenographists have called Manilius, and represents it as it may be seen on the eighth 
day of the lunation. It will be observed that a deep, black shadow is cast iciOtin the 
left-hand side of the circle, and without its right-hand side; and that an isolated peak 
arises from the centre of the cavity, casting a conical shadow in the same general direc- 
tion. The length of the shadow inside the cavity, compared Avith that on the outside, 
indicates that the central cavity is far below the general surface of the Moon; that, in 
other words, the circular ridge encloses a cavity, hollowed out into the lunar substance. 
This circular ridge, called Manilius, rises 7600 feet above the general surface of the Moon. 

Q. Are there many of these circular depressions on the Moon's surface ? 

A. In many parts the Moon's surface is comj)letely studded 
with them. No less than 148 large cavities of this description 
have been measured, besides which there are numerous small ones. 

Q. ^Miat is the breadth of these lunar cavities ? 

A. Most of them are from one to ten miles in diameter ; but 
some are much larger. 

Manilius, sketched \r\ fig. 6, (Lunar Shadows,) is about 25 miles wide. Fig. 11 gives 
the appearance of a large cavity, called Clavius, on the tenth day of the lunation. This 
cavity measures 143 miles across. It will be observed that there are several secondary 
cavities opened out on its floor, and that the floor is surrounded by an irregular ridge, 
rising in this instance to the height of 18,000 feet. This height is indicated by the long 
shadow cast within the border of the cavity. Fig. 5 shows the foreshortened elliptical 
form of a cavity, called Humboldt, situated near the right edge of the Moon, as it is seen 
on the sixteenth day of the lunation. A central peak rises within the ring-shaped ridge ; 
and two small black shadows thrown across the left edge of the cavity show that high 
peaks tower on the opposite side. 

Q. Are the shadows, cast behind the lunar elevations, seen to shorten as the 
Sun rises upon them ? 

A. Yes. When the peaks and ridges first come into light, they 
have very long shadows beyond them ; but the shadows then grow 
shorter and shorter, until at last they disappear. 

Fig. 2 (Lunar Shadows) shows the appearance of the shadows cast behind the ter- 
minal peaks of the ridge called the Apennines, and within and behind the cavities called 
Antolycus and Aristillus, on the seventh day of lunation. Fig. 1 gives the appearance 
of the same object twenty-four hours after. It will be observed that the shadows behind 
the peaks have been shortened by more than three-fourths ; and the shadow inside the 
circular ridges is so far withdrawn that half the contained plain is uncovered. In Aris- 
tillus, the summit of a central peak is seen, just rising beyond the shadow. 

In Jig. 7 (Lunar Shadows) the shadows of the large cavity called Tycho are repre- 
sented as they may be seen six hours before the completion of the ninth day of the luna- 
tion. The chief part of the internal depression then lies in shadow; but a central peak 
rears its summit through the shadow, high enough to catch the sunbeams. To the right, 
the circular ridge throws its outer shadow; but a few elevated peaks are already in 
light beyond. Fig. 8 gives the appearance of the same object four hours later; the iii- 
ternal shadow is withdrawn from the greater portion of the floor of the cavity, and the 
central peak is entirely revealed, with its own shadow cast upon the illuminated portion 
of the floor of the cavity. Fig. 9 represents the appearance of the same (Tycho) on the 
tenth daj' of the lunation. All the shadows are here very much diminished ; and one 
or more smaller peaks are visible, standing near the base of the larger central one. 
This cavity is 55 miles wide; its circular ridge is 17,000 feet high; and the larger cen- 
tral peak cannot have less elevation than one mile. 

Q. Do the lunar shadows decrease as quickly as the terrestrial shadows, when 
the Sun is in the act of rising upon the objects which cast them ? 

A. No ; they require a period twenty-nine times greater than 



THE SATELLITES. 



85 



the terrestrial shadows, because the Moon revolves on her axis so 
slowly as to make the lunar day equal to twenty-nine of our days. 
Therefore, her shadows decrease proportionably slower than those 
on the Earth. 

In Jig. 19 (Lunar Shadows) is a cavity named Petavius. as seen on the seventeenth 
day of the lunation. The serrated tops of the left-hand border of the ridge may be noticed 
in light, while all the cavity and outer surface on that side are yet in darkness. The 
central peak terminates in several broken summits, already illuminated. 

Q. Are the shadows always seen on the same sides of the lunar elevations? 

A. No. Before the full moon the shadow is cast on the left 
side of the elevations, that is, towards the east ; and after the 
full moon it falls on the right side, or towards the west. 

Fig. 15 (Lunar Shadows) represents the shadows cast behind certain projecting ob- 
jects, near a cavity bearing the name of Thebit, on the eighth day of the lunation. It 
will be observed that there is in both sketches a long ridge, crossing the centre of a sort 
of oval plain. In Jig. 15 a deep shadow is cast by this declivity. In Jig. 16 its surface 
lies in bright sunshine. A small cavity to the right of the ridge casts its shadow awnr/ 
from the ridge in Jig. 15, but upon it in Jig. 16. In the two cases the shadows are re- 
versed. In Jig. 15 the floor of the large cavity Thebit, marked g, is half illuminated; 
in Jig. 16 it is in entire darkness, but the highest peaks of its ring-shaped ridge have 
just caught the sunshine. 

Q. Do the Moon's circular ridges bear any resemblance to the volcanic cra- 
ters on our Earth ? 

A. They do not; they are much more vast, and are charac- 
terized by entirely different features. 
Q. What is a terrestrial volcanic crater ? 
A. The aperture or mouth of a terrestrial volcano. 

Q. What do the lunar cavities resemble ? 

A. Circular pits of large size, excavated in the Moon's surface, 
and fringed with elevated rugged edges. 

Terrestrial volcanic craters are for the most part situated on the summits of elevated 
peaks; while the lunar craters, as they are sometimes called, are mere depressions, or 
surface cavities. 

Q. Can any opinion be formed of the probable difference of the forces that 
have given rise to mountain projections in the Earth and in the Moon? 

A. The terrestrial projections seem to have been forced up by 
powers acting at a great distance beneath the surface, and ex- 
tending over a large space. The annular ridges of the Moon 
seem to have been formed by forces that have had a very confined 
range, both as regards depth and extent. 

Q. Are there no instances on the Moon's surface in which an extensive area 
has been disturbed during the production of a crater? 

A. There is one isolated instance in which such extensive dis- 
turbance appears to have taken place. 
Q. What is the name of this isolated object? 

A. It is called Tycho. There are cracks radiating from it, 
extending in one direction at least seventeen hundred miles. 
Traces of these cracks are visible about the time of full moon. 

Fig. 10 (Lunar Shadows) represents the appearance of these cracks around Tycho the 
day after full moon. Fig. 12 shows the crater Copernicus as it appears on the tenth 



86 bouvier's familiar astronomy. 

day of the lunation, with its basket-worklike concentric ridges, and its three isolated 
peaks. The other is the crater Kepler, sketched in Jig. 13, as it may be seen on the 
twelfth day of the lunation. Copernicus is 55 miles in diameter, and its circular ridge 
is above 11,000 feet high. The crater of Kepler is 22 miles in diameter, and its sides 
tower 10,000 feet above the internal floor. 

Q. Are there no other kind of mountainous elevations upon the Moon's sur- 
face besides these circular ones ? 

A. There are examples both of extended ridges and isolated 
peaks without any surrounding ring ; but these are of rare occur- 
rence, when compared with the number of the ring-shaped ridges. 

Q. What does the telescope show to be the nature of those portions of the 
lunar surface that are of inferior brilliancy? 

A. It shows that they are level tracts of the nature of plains. 

Q. How MANY of these comparatively level plains are there on the visible hemi- 
sphere of the Moon ? 

A. Thirteen well-marked plains have been observed. 

Q. What is the general extent of these plains in the Moon ? 

A. They vary in breadth from two hundred to nearly eight 
hundred miles. 

Q. Are the lunar plains surrounded by ridges ? 

A. Yes : they are encompassed by borders or ridges somewhat 
like the cavities or craters. 

Fig. 14 (Lunar Shadows) represents the appearance of one of these lunar plains as 
it may be seen on the eleventh day of the lunation, with its surrounding ridges and 
cavities. This plain has been called Mare Humorum. In Jig. 18 is sketched the ap- 
pearance of another lunar plain, (Mare Crisium,) as seen on the nineteenth day of the 
lunation. It will be observed that its elevated margin may be perceived projecting 
out quite into the dark part of the lunar sphere, and catching the Sun's rays before the 
neighboring plain can receive them. Mare Crisium is 280 miles in breadth, and 360 in 
length. 

Q. What were these more level districts once supposed to be ? 

A. They were once supposed to be oceans and seas; hence 
they have individually received the appellation of Mare — a sea. 

Q. How can it be ascertained that they are not oceans and seas ? 

A. They are all of them covered hj fixed irregularities of sur- 
face ; whereas water, at that distance, would present a smooth, 
even aspect. 

Q. Are there any clouds floating round the lunar surface ? 

A. No clouds have ever been detected by the most powerful 
telescope. 

Q. How is it known that the Moon has not any clouds surrounding it? 

A, The elevations and depressions on the Moon's surface are 
always seen clearly, provided our own atmosphere is cloudless. 

Q. If a VAIL OF CLOUDS wcrc floating round the Moon, what would be the 
effect ? 

A. The irregularities of its surface would be at times concealed^ 
though our own atmosphere were cloudless. 

Q. Is there any water on the Moon ? 

A. In all probability there is not. 



THE SATELLITES. 87 

Q. Upon what grounds can it be concluded that the lunar surface is without 

V? \TFR ? 

A. When very faint stars are seen near the edge of the Moon, 
their light is undimmed, 

Q. What would be the effect if there were water on the lunar surface ? 

A, If there were water on the Moon, it must be converted into 
vapor where the Sun's heat is experienced continuously for half 
a month. 

Q. Has the Moon any atmosphere ? 

A. The Moon cannot have any atmosphere of sufficient density 
to be worthy of the name of air. Some astronomers suppose the 
Moon has an atmosphere of extreme rarity^ compared with that of 
the Earth. 

Q. How can it be ascertained that the Moon is without an atmosphere ? 

A. The light of planets and stars never varies when passing 
near the edge of the Moon, as it would do if there were a lunar 
atmosphere. 

Q. Is there, then, any diffused daylight upon the Moon ? 

A. The bodies on the Moon's surface are either in brilliant 
sunshine or in black darkness. There is no gradation of light 
between the two, because the atmosphere of the Moon (if any) is 
not sufficiently dense to refract the rays of the Sun. 

Q. Can there be any variations of daylight on the Moon ? 

A. No ; the sky must seem black at noonday, and the scorch- 
ing beams of the Sun pour down upon a surface which is never 
fanned by a breeze^ or moistened by a shower^ or screened by a 
cloud. 

Q. Is the absence of an atmosphere perceptible when the Moon is viewed 
through a telescope ? 

A. Yes ; the want of the softening influence of atmospheric 
refraction is at once discerned. 

Q. What appearance does this absence of an atmosphere present ? 

A. The illuminated portions of the Moon gla?'e in the bright 
sunshine, while the obscured parts are covered with shadows traced 
in the sharpest and boldest outlines. 

Q. Can there be any life on the Moon's surface ? 

A. Certainly no such life as exists upon our globe. 



SECTION II. 

lupiltr's Satellites. 

Q. What are the minute bodies which the telescope reveals as attendant upon 
Jupiter ? 

A. They are Jupiter's satellites or attendant moons. They are 
to Jupiter what our Moon is to us. 



88 bouvier's familiar astronomy. 

Q. How MAXY attendant moons has Jupiter ? 

A. Jupiter hsLS four moons. 

•'Beyond tlie sphere of Mars, in distant skies, 
E/evolves the mighty magnitude of Jove 
With kingly state, the rival of the Sun. 
About him round fods planetary moons. 
On earth with wonder all night long beheld. 
Moon above moon, his fair attendants dance." 

Q. Do these moons kevolve around Jupiter ? 

A. Yes; they move round Jupiter as our Moon revolves round 
our Earth. 

Q. Are the orbits of Jupiter's moons elliptical ? 

A. Their orbits are almost circles, having a veri/ small eccen- 
tricity. 

Q. Do they revolve round the planet from west to east ? 

A, They do. 

Q. Are the planes of Jupiter's satellites ixclixed to the plane of his orbit ? 

A. The planes of Jupiter's satellites coincide nearly, though 
not exactly, with the plane of the planet's orbit. 

Q. When and by whom were the satellites of Jupiter discovered ? 

A. They were discovered in the year 1610 by Galileo. 

Q. How do Jupiter's satellites appear when viewed through the telescope ? 

A. They appear like small stars ranged nearly in a line with 
the planet's equator. 

Q. What are the dimensions of Jupiter's satellites ? 

A. They are, all of them, somewhat larger than our Moon. 

Q. How are the four satellites designated ? 

A. The one which revolves nearest to the planet is called the 
first satellite; the one next in order, the second; and so on. 

Q. Which is the labgest of Jupiter's moons ? 

A. The third satellite from the planet is the largest, measuring 
3580 miles in diameter, which is 440 miles greater than that of 
the planet Mercury. 

Q. Which is the smallest of Jupiter's satellites ? 

A. The second in order from the planet is 2190 miles in dia- 
meter, being veri/ little larger than our Moon. 

Q. Are these moons as labge in comparison to the size of Jupiter as our Moon 
is, compared with the size of our Earth ? 

A. No ; they are much smaller bodies, viewed relatively to the 
size of Jupiter, than our Moon is, compared with our Earth. 

Q. What is the diffebence between the diameter of Jupiter's largest satellite 
and the planet itself? 

A. The largest satellite has only one twenty-fifth part of the 
diameter of Jupiter, while our Moon has nearly onefourth the 
diameter of the Earth. 



THE SATELLITES. 



89 



Q. How FAR is the nearest satellite from the body of Jupiter ? 

A. The nearest satellite is about two hundred and sixty-six: 
thousand miles from the planet. 

Q. How FAR is the farthest satellite from Jupiter ? 

A. The farthest satellite is nearly a million and a quarter of 
miles distant from Jupiter. 

Q. How LONG does it require the first or nearest satellite to revolve round 
Jupiter ? 

A. The first satellite revolves round its primary in one day 
and eighteen hours. 

Q. In what time does the fourth or farthest moon revolve round its primary ? 

A. The fourth satellite revolves round Jupiter in rather less 
than seventeen days. 

Q. AVhat size does the nearest satellite appear from Jupiter? 

A. The first satellite must appear to the inhabitants of Jupiter 
a little larger than our Moon ; the second and third appear one- 
third less, and the fourth about one-fourth as large as our Moon. 

Q. Do the satellites of Jupiter revolve on their axes as the Moon does ? 

A. All of them are supposed to rotate upon their axes in the 
same time in which they revolve round their primary : conse- 
quently, the inhabitants of Jupiter can never see but one hemi- 
sphere of each moon. 

Q. Do the satellites of Jupiter suffer eclipse as our Moon does ? 

A. Yes ; by watching the moons of Jupiter, eclipses may be 
seen resembling those of our Moon. 

Q. Do eclipses of Jupiter's moons occur more frequently than with our Moon ? 

A. They do ; the three nearest satellites suffer eclipse at every 
revolutio7i round the planet. 

Q. What important discovery have astronomers made by observing the eclipses 
of JupitPT-'s satellites ? 

A. The aberration of light ;'^ and that a ray of light travels at 
the rate of nearly two hundred thousand miles in a second of 
time. {See Note 21.) 

"By these observed, the rapid progress finds 
Of LIGHT itself: how swift the headlong r;ty 
Shoots from the Sun's height through unbounded space ! 
At once enlightening air, and earth, and heaven." 

Roemer, a Danish astronomer, in 1675, discovered that light required time to travel 
through space. Before that time it was thought to be propagated instantbj. Subse- 
quently, Bradley confirmed Roemer's theory by the discovery of the aberration of light. 
Q. Do the satellites of Jupiter exhibit the same phases which our Moon does 'r 
A. They do ; as the first satellite revolves about Jupiter in one 
day and eighteen hours, or in forty-two hours, in that short spact, 
of time it exhibits all the phases from a slender crescent to a full 
moon. 



* See Part iii. chap. i. sec. i. div. iv. 



90 bouvier's familiar astronomy. 

Q. Why do the satellites of Jupiter move so much more rapidly in their orbits 
than our Moon does in hers ? 

A. Because the attractive force of Jupiter is so great, that if 
his satellites had not a rajoid projectile motion to overcome his 
centripetal for ee^ they would fall to the body of the planet. 

(See centrifugal and centripetal forces, Part i. chap. i. sec. iii. div. ii.) 

SECTION III. 

%\^t ^ings anir p[oons of Saturn. 

Q. What does the telescope reveal with regard to the planet Saturn ? 

A. It shows him encompassed by hroad rings, and attended by 
eight satellites or moons. 

"One raoon to us reflects its chfeerful ligit, 
There, eight attendants brighten up the night; 
Here, the blue firmanent bedecked -with stars. 
There, overhead a lucid arch appears." 

Q. What are the rings which encircle Saturn ? 

A. They are extremely thin, opaque bodies, which surround 
the planet. 

Q. How is it KNOWN that these rings are opaque ? 

A. Because they cast a shadow on the body of the planet on 
the side nearest to the Sun, and on the other side receive the sha- 
dow of Saturn. 

Sir William Herschel was the first who discerned the shadow of Saturn's ring on the 
planet when the ring itself was invisible, having its thin edge turned towards us. Place 
one edge of a slip of paper against the wall, so that the other edr/e may be in a line with 
your eye; at a little distance, the edge or thickness of the paper would be invisible, be- 
ing too minute to be distinguished ; but its shadow, if the light were thrown on its upper 
or under surface, would be readily seen projected on the wall. Saturn's ring being so thin, 
could not be perceived through the telescope, yet the shadow it cast, being broad, could 
easily be distinguished on the body of the planet. 

Q. How MANY RINGS has the planet Saturn? 

A. One outer or luminous ring, and an inner dark ring, which 

reflects but little light. 

Q. What is the nature of this outer or luminous ring. 

A. It seems like an opaque substance, capable of reflecting 
light; and by the aid of good telescopes appears to be divided into 
three separate rings, the inner one of which is more luminous 
than the others. [See Note 22.) 

Q. Of what nature is the inner dark ring ? 

A. It appears to be composed of matter less opaque than the 
luminous rings, and less capable of reflecting light. (See Note 23.) 

Q. How are these rings sustained in their places, and prevented from falling 
on the body of the planet ? 

A. They rotate or revolve very rapidly round the body of Sa- 
turn. It is the centrifugal force arising from this rotation which 
sustains them in their position. 



THE SATELLITES. 



91 



Q. What TIME do the rings of Saturn require to make one revolution ? 

A. The times of the revolutions of the different rings are not 
certainly known. But it is supposed they revolve in different 
periods ; those nearest to the planet in all probability move with 
somewhat greater velocity than those more distant from it. 

The rings are supposed to rotate in about the same time the planet performs a revolu- 
tion on its axis, 

Q. At what DISTANCE are Saturn's rings from the body of the planet ? 

A. The inner ring is about nineteen thousand miles from the 
surface of Saturn. 

Q. What is the thickness of the rings ? 

A. They are not more than from one hundred to two hundred 
miles in thickness. 

Q. Of what USE are the rings of Saturn ? 

A, They reflect the light of the Sun ; and being so much nearer 
to the planet than our Moon, and of such vast magnitude, they 
would tend to enlighten the planet by one vast arah of moonlight. 

Q. Do the rings of Saturn always present the same appearance through the 
telescope ? 

A. No ; sometimes they appear in the form of an ellipse^ which 
contracts until it dwindles into a straight line. 

Q. Does Saturn's ring always remain parallel to itself during its revolution 
round the Sun ? 

A. It does; as may be seen in the following figure. 

Fig. 65. 




Let a 6 be the orbit of the Earth, and ABC, Ac. represent Saturn in different parts 
of his orbit. At the point C, the ellipse appears the most open to an inhabitant of the 
Earth, and contracts until it comes to E, where it presents only a line to the eye of a 
spectator on the Earth ; from E to G it opens again, and at G appears as broad as at C. 
From G it contracts, till at A it has only the appearance of a line ; from A to C it again 
appears to open. 

Q. How often do we lose sight of Saturn's ring, owing to its plane being in 
the same line with our eyes ? 

A. Twice in each of Saturn's revolutions round the Sun ; that 
is, once in nearly every fifteen pears. 

In the following figure, four of the different views of Saturn's ring nre given: from 
the open ellipse to the disappearance of the ring. 



92 



BOUVIER S FAMILIAR ASTROXOMY. 

Fis. 66. 




No. 1 represents when the ring has disappeared, or when it is at the points A an<l E 
oifig. 65. No. 2 shows the ring as a mere line of light; No. 3 exhibits the upper side 
of the ring as when it is at C, and No. 4 the lower side of it, as when it is at G, in the 
same figure. 

Q. How MANY attendant moons has the planet Saturn? 

A. Saturn has eight satellites or moons. 

Q. Can Saturn's moons be seen by telescopes of moderate power ? 

A. Only one of Saturn's moons is large enough to be seen by 
telescopes of moderate power. This moon is known by the name 
of Titan. [See Note 24.) 

Q. Do Saturn's moons suffer eclipses ? 

A. They do; but it requires very powerful telescopes to dis- 
cern them. 

Q. Has any relation been discovered between the periods of any of Saturn's 
satellites ? 

A. Yes ; Sir John Herschel discovered that the period of the 
revolution of the first satellite in order from the planet is half 
that of the third; and that the period of the second in order is 
half that of the fourth. 

Q. In what time does the first satellite revolve round Saturn ? 

A. In about tiventy-two hours and a half^ and the third re- 
quires double that time, or forty-five hours. 

Q. How LONG does the second satellite require to perform its journey round 
the primary ? 

A. The second satellite revolves about its primary in a little 
less than thirty-three hours ; and the fourth requires double that 
time, or less than sixty-six hours. 



THE SATELLITES. 



93 



Q. What is the distance of Saturn's farthest satellite from the planet ? 

A. It is situated at the distance of about two millions three 
hundred thousand miles from Saturn, or nearly ten times the dis- 
tance of our Moon from us. 

Q. What kind of appearance do the heavens present to the inhabitants of 
Saturn ? 

A. The celestial scenery on Saturn must be magnificent beyond 
description. The ring rolling round its primary, reflecting light 
from its surface similar to that of our Moon ; besides which, 
eight moons exhibit their phases, from the slender crescent to the 
full orb. 

Q. Are any of Saturn's moons nearer to it than our Moon is to us ? 

A. Three of Saturn's moon's are much nearer to him than our 
Moon is" to us. 

Q. Do Saturn's moons exhibit phases like our Moon ? 

A. Yes. The satellite nearest to Saturn is not more than one- 
half as far from it as our Moon is from us ; and in the course of 
twenty-two hours and a half exhibits all the phases which our 
Moon performs in a month. 

A list of Saturn's moons, according to the nomenclature of Sir John Herschel, the 
dates of their discovery, and the names of their discoverers, is given below : 

1. Mimas, Sept. 17, 1789, Sir W. Herschel. 

2. Enceladus, Aug, 19, 1787, " " 

3. Tethys, March, 1684, Dominic Cassini. 

4. Dione, March, 1684, " " 

5. Rhea, Dec. 23, 1672, " " 

6. Titan, March 25, 1655, Huyghens. 

7. Hyperion, Sept. 18, 1848, Bond and Lassell. 

8. Japetus, Oct. 1671, Cassini. 

Hyperion, the seventh satellite, has been recently discovered by Professor Bond, of 
Cambridge, Massachusetts, and on the same night by Mr. Lassell, of Liverpool. 



SECTION IV. 



%\z Pooits of Uranus. 

Q. How MANY satellites has the planet Uranus ? 

A. It is supposed to have six attendant moons; but owing to 
the great distance of the planet from us, four only can be detected 
by the most powerful telescopes, and under the most favorable 
circumstances. 

Q. Are there any peculiarities observed with regard to the motions of the 
satellites of Uranus ? 

A. Yes ; they revolve round him in orbits nearly perpendicu- 
lar to the plane in which the planet moves round the Sun ; and 
unlike all the other known planets and satellites, which move from 
west to east, they move in a reverse direction, that is, from east 
to west. 



94 bouvier's familiar astronomy. 

Fig. 67. 



Let AR represent the orbit of the Earth, the line M E will show the inclination of the 
orbit of our Moon to that of the Earth. Now, if A R represent. the orbit of Uranus, 
the line V S will show the inclination of the orbits of his satallites, which is 78° 5S' to 
the plane of the ecliptic. 

Q. Are the orbits of the moons of Uranus elliptical, like those of the other 
planets and satellites ? 

A. No ; the orbits of the moons of Uranus are nearly circular. 

The moons of Uranus, according to Sir "William Herschel, are six in number ; but /our 
only have been seen by Sir Joha Ilerschel, Mr. Lassell, and Otto Struve, namely : 

1. Ariel, which has a period of 2 days 12 hours. 

2. Umhriel, " " 4 '• 4 " 

3. Oberon, « " 10 " 23 « 

4. Titania, « " 13 " 11 " 



SECTION V. 

STbe IJloons of §^cptane. 

Q. Has Neptune any satellite ? 

A. One attendant moon has been discovered as belonojinor to 
this planet, and it is believed that another has been detected. 

Some astronomers have thought they have been able to distinguish a ring round this 
planet; but as yet no satisfactory observations have been made with regard to it. 

In the following figure, No. 1 represents Jupiter and three of his moons, at their 
respective distances from their primary. One end of a string placed on the centre of 
the planet, and extended seven inches, would designate the distance of the orbit of the 
fourth and most distant satellite. No. 2 is Saturn, with the relative distances of the 
orbits of six of his satellites. One end of a string placed on the centre of Saturn, and 
extended seven inches, would reach the orbit of his seventh satellite; while it would 
require a string of sixteen inches to reach to the orbit of the eighth and most distant 
of Saturn's moons. No. 3 represents the planet Uranus, and his relative size with re- 
gard to Jupiter and Saturn. The orbits of five of his satellites are designated, but, for 



THE SATELLITES. 



95 



want of space, the sixth must be shown by a string eleven inches in length, one end 
ft" which is placed at the centre of the planet, and the other extended its full length. 
No. 4 represents the planet Neptune and his first satellite. No. 5 is our Earth and 
Moon, drawn to the same scale. 

Fig. 68. 




96 bouvier's familiar astronomy. 

CHAPTER VII. 

llDti0ti of tfee ^art^. 

Q. Does the Earth always remain in the same part of space ? 

A. It does not. It is constantly changiyig its position with 
great velocity. 

Q. How MANY motions has the Earth ? 

A. Its principal motions are two^ the one round its axis from 
west to east, and the other round the Sun. 

"Froin -west lier silent course advance 

With, inoffensive pace, that spinning sleeps 

On her soft axle, while she paces even 

And bears thee soft, with, the smooth air along." — Milton. 

Q. What causes the Earth to move rouxd the Sun ? 

A. It is acted upon by two forces, which, when combined, cause 
it to revolve round the Sun. 

Q. What are the two forces which act together to cause this motion of the 
Earth round the centre of the system ? 

A. The one is a power seated in the Sun, called the attraction 
of gravitation, or centripetal force ; the other the centrifugal force, 
or the Earth's projectile motion. 

"And if each system in gradation roll. 
Alike essential to the amazing whole, 
The least confusion but in one, not all 
That system only, but the whole must fall. 
Let Earth unbalanced from her orbit fly. 
Planets and suns run lawless through the sky." — Fopb. 

SECTION I. 

^mrttal potions of t\t ©art^. 

Q. What EFFECT is produced by the rotation of the Earth on its axis ? 

A. It produces the succession of day and night. 

A day in common language is the interval of time which elapses from the rising to 
the setting of the Sun. But astronomers reckon from noon to noon : thus, June 3, 20 
hours astronomical time, would be June 4, 8 o'clock in the morning civil time, or accord- 
ing to the common reckoning. 

Q. How can the rotation of the Earth produce daylight and darkness ? 

A. As the Earth revolves on its axis it always presents one 
hemisphere to the Sun's rays ; and to the inhabitants of that part 
of the Earth turned towards the Sun it is day. 

"Blest power of sunshine! genial day! 
Wliat balm, -what life is in thy ray." — Lalla. Rookh. 

Q. What causes the phenomena of sunrise and sunset? 

A. As the Earth turns on her axis, to the inhabitants of that 



MOTION OF THE EARTH. 



97 



point on her surface which is just receiving the first rays of the 
tSun, the Sun is just rising ; while to the inhabitants of that part 
which is just receiving his last rays, he is setting. 

Q. When is it midday ? 

A, When the centre of the Sun is on the meridian of any place ^ 
it is midday or noon to the inhabitants of that place. 

Q. When is it midnight ? 

A, When the .centre of the Sun is on the meridian of the ob- 
server extended into the opposite hemisphere. Thus, midnight is 
just twelve hours from noon. 

It is midnight to us when it is noon or mid-day to our antipodes. 

"Day takes his daily turn, 
Rising between the gulphy dells of night, 
Like ^vhitened billows on a gloomy sea." — Joa.nna Baillce. 

Fig. 69. 



Let G R C represent the Earth, which is 
revolving on her axis from G to R, R to C, 
and let the Sun be fixed in the heavens at 
Z; it will illuminate all that part of the 
Earth which is above G C. To the inhabit- 
ants at G, the western boundary of sunlight, 
the Sun will be just rising; and when, by 
the motion of the Earth, they arrive at R, it 
will be noon, while it will be setting when 
they arrive at C. As only one-half the 
heavens can be seen at any one time, the 
spectator at G will see the concave hemi- 
sphere Z N ; consequently, the Sun's place 
will be the eastern boundary of his horizon. 
When, by the rotation of the Earth, the ob- 
server at G arrives at C, the boundary of his 
horizon will be N H Z ; consequently, the 
Sun will be in the western boundary of his 
horizon, and will appear to be setting. 



Q. What is the horizon ? 

A. The horizon is that circle in the heavens which hounds the 
view on all sides, and which is greater or less as the observer is 
more or less elevated from the surface of the Earth. 

Q. What is the horizon called which bounds the observer's view on all sides ? 

A. It is called the sensible horizon. 

Q. Is there any other horizon except the sensible horizon ? 

A. There is. An imaginary line drawn through the centre of 
the Earthy parallel to the sensible horizon, and extended to the 
heavens, is also called a horizon. 

Q. What is that horizon called which is bounded by an imaginary line drawn 
through the centre of the Earth ? 

A. It is called the rational horizon. 




In fiq. 69, to an inhabitant at R, his rational horizon would 
line H b. 



bounded by the 



98 BOUVIER'S familiar ASTRONOxMY. 

Q. What is that point of the horizon called which is opposite to an observer's 
face stationed north of the tropics, when he stands looking towards the Sun at 
noon? 

A. To any observer stationed north of the tropics, that point 
of the horizon towards which the Sun is situated at noon, is 
called the south point of the horizon. 

Q. What is that point of the horizon called which is directly behind an ob- 
server stationed north of the equator, when he stands looking towards the south ? 

A. That point of the horizon which is directly opposite to the 
south, is called the north point of the horizon. 

Q. Which are the east and west points of the horizon ? 

A. Those two points of the horizon which are midway between 
the north and south points. 

Q. How are the east and west points of the horizon situated with regard to 
each other ? 

A. Like the north and south points, the east and west points 
are directly opposite to each other. 

Q. When an observer stands with his face to the south, in what direction are 
the east and west points with regard to his position ? 

A. The east is to his left hand, and the west to his right hand. 

Q. What are these four points of the horizon called ? 

A. They are called the cardinal points of the horizon. 

Q. How far are the four cardinal points asunder by angular measurement ? 

A. As the horizon is a circle, it must contain 360 degrees ; 
and as the four cardinal points of the horizon are at equal dis- 
tances from each other, it follows that the east, west, north, and 
south points must be situated 90 degrees, or a quarter of a circle, 
from each other. 

Q. What is the zenith ? 

A. The zenith is that point in the heavens immediately over the 
head of the observer. 

Q. What is meant by the azimuth of a body ? 

A. In order to find the azimuth of a body, an imaginary line 
must be drawn from the zenith (that is, the point overhead) 
through the body, and extended until it touches the horizon. The 
angular distance between the north or south points of the horizon, 
and the point where the imaginary line meets the horizon, is the 
azimuth of a body. 

Q. What is the azimuth of a heavenly body due west from the observer ? 

A. An imaginary line drawn from the zenith through the body 
would touch the west point of the horizon ; now, the angular dis- 
tance from the north or south points to the west point of the 
horizon is just 90 degrees ; therefore the body must have an azi- 
muth of 90 degrees. 

Sir John Herschel says — "Azimuth may be reckoned eastward or westward from the 
north or south point, and is usually so reckoned to 180° either way. But to avoid con- 
fusion, and to preserve continuity of interpretation when algebraic symbols are used, 



MOTION OF THE EARTH. 



99 



(a point of essential importance, hitherto too little insisted on,) we shall always reckon 
azimuth /row the points of the horizon most remote from the elevated pole westward, (so 
as to agree in general directions with the apparent diurnal motion of the stars,) and 
carry its reckoning from 0° to 360°, if always reckoned positive, considering the east- 
ward reckoning as negative." 

Q. What is meant by a meridian ? 

A. The meridian of any observer is an imaginary line drawn 
from the north point of his horizon, through the zenith, to the 
south point of his horizon. 

Q. What is meant by the altitude of a star or planet ? 

A. The angular distance of any heavenly body from the hori- 
zon is its altitude. 

Q. How is this angular distance reckoned ? 

A. An imaginary line is drawn from the zenith through the 
body, and extended to the horizon. The number of degrees mea- 
sured on this imaginary line, from the point where it meets the 
horizon to the body, shows its altitude. 

Fig. 70. 



Let C be the place of the observer, H Z N 
would represent the sphere of the heavens, the 
dotted circle H the horizon, P P the poles of 
the celestial sphere, and Z the zenith of the 
observer. 

Now, in order to find the altitude and azi- 
muth of the star S, an imaginary line must be 
drawn from the zenith Z, through the star S, 
to the horizon. The arc of the horizon H A is 
its azimuth, and the angular distance from the 
point A to the star S is its altitude. 



Q. Why do the heavenly bodies seem to move from east to west in the con- 
cave sphere of the heavens ? 

A. Because the Earth's surface on which the observer stands is 
carried along in the opposite direction., that is, from west to east, 
in consequence of the Earth's rotation. 

Q. To what is the apparent motion of the heavens due ? 

A. The apparent motion of the heavens is owing to the real 

motion of the Earth in the opposite direction. 

Q. How FAR do the heavenly bodies appear to have moved after the lapse of 
an hour ? 

A. The heavenly bodies pass ih.xo\x^ fifteen degrees of angular 
measurement in one hour ; that is, through a quarter of a degree 
in one minute. 

As the Earth revolves on its axis once in twenty-four hours, that is, through 360° in 
twenty-four hours, it must revolve through the twenty-fourth part of 360° in one hour. 
The twenty-fourth part of 360° is 15°, which is the angular distance the Earth moves 
in one hour. 




100 bouvier's familiar ASTROIS'OMY. 

Q. Did the ancients believe that the Earth revolves on its axis ? 

A. No; they conceived the Earth to be stationary/, and the 
Sun and stars to revolve round it once in twenty-four hours. 

As the Sun and stars are at such vast distances from us, they ttouM be required to 
revolve vrith inconceivable rapidity to perform a diurnal rotation round the Earth. 
Q. Why do the Sun and stars appear to move around us ? 

A. It is owing to the revolution of the Earth on its axis. 

Q. In -what length of time do the Sun and stars appear to move around us ? 

A. In the same time it takes the Earth to turn once on her 
axis; that is, once in twenty-four hours. 

Q. Is the time -which the Earth requires to perform a revolution on its axis 

INVARIABLE ? 

A, It is. La Place has shown that the length of the day has 
not varied the hundredth part of a second since the observations 
of Hipparchus two thousand years ago. 

Q. Are the stars scattered all over the firmament ? 

A. They are, 

Q. Then -why can we not see the stars in the daytime ? 

A. Because the Sun's light is so much more fowerfxd, that it 
obscures the fainter light of the stars — 

" Confounded and outdone 
By the superior lustre of the Sun.' 

Q. When can we see the stains ? 

A. When that part of the Earth on which we live is turned 
aivay from the Sun's rays, we can then see the stars. 

Q. Can the stars ever be seen by the naked eye in the daytime ? 

A. Yes. By descending the shaft of a very deep mine, the 
stars may be seen at noon as well as at night ; or during the ob- 
scuration caused by a total solar eclipse, the brightest stars are 
visible. 

By the aid of a telescope, stars are rendered visible even in the vicinity of the Sun ; 
that is, when apparently near to the Sun's orb, though they are in fact immensely dis- 
tant from, yet in the same direction with, him. 

SECTION II. 

giniuial Potion of l^c 6arl^. 

"Go! all the sightless realms of space survey; 
Returning, trace the planetary way ; 
The Sun that in his central glory shines. 
While every planet round his orb inclines ; 
Then at our intermediate globe repose, 
And view yon stellar satellite that glows. 
Or cast along the azure vault thine eye. 
When golden day enlightens all the sky; 
Around, behold Earth's variegated scene. 
The mingling prospects and the flowery green. 



MOTION OF THE EARTH. 



101 



The mountain's brow, the long-extended -woGd, 
Or the rude rock that threatens o'er the flood; 
And say, are these the -wild effects of chance ? 
Oh! strange effect of reasoning ignorance!" — Botse. 

Q. Does the Sun move in the heavens ? 

A, No ; but owing to the motion of the Earth round the Sun, 
he appears to move round the Earth ? 

If a person be seated in a boat on smooth water, and suffer himself to be carried along 
by the current, without the use of oars, he would not feel the motion of the boat. Now 
let him fix his eye on the shore, and all the objects appear to be in motion. Thus, the 
Sun, although stationary with regard to the Earth, appears to move. 

Q. How LONG does it require the Earth to move ro«.nd the Sun ? 

A. The Earth makes one revolution round the Sun in a year ; 
hence it is called the Annual Motion of the Earth. 

Q. How is it known that the Earth has completed one revolution after this 
lapse of time ? 

A. If the Sun be seen in a line with a fixed star at any particu- 
lar day and hour, it will be found to be in a line again with the 
same star in one year ; consequently, the Earth must have gone 
once round the Sun in that time. 

Q. How can it be ascertained that the Earth is between certain stars and 
the Sun ? 

A, By observing that those stars are exactly on the meridian 
at midnight. 

Q. If an observer look at a star on the meridian at midnight, how is he situ- 
ated with respect to the Sun ? 

A. The Sun is on the meridian to the inhabitants of the oppo- 
site side of the Earth. 

Fig. 71. 




Let S {fig. 71) represent the Sun's place in the great concave of the heavens, and M 
the place of some star directly opposite to the Sun in the apparent surface of the celes- 
tial sphere j let E represent the Earth half in daylight and half in darkness. Then it 



102 bouvier's familiar astronomy. 

is clear that the star must be exactly opposite the middle of the dark half of the Earth 
(that is, it must be on the meridian at midnight) when the Sun is exactly opposite to 
the other half of the Earth. So to the inhabitants at a it is midnight, while to those at 
h it is noon. Now, to show that the Sun is precisely on the opposite side of the Earth 
to the star, is to prove that the Earth is between the Sun and the star. 

Q. What is the complete period of time called in -which the Earth makes one 
revolution round the Sun ? 
A. It is called a year. 
Q. AVith what velocity does the Earth move in her orbit round the Sun ? 

A, The Earth moves at the mean rate of sixty-eight thousand 
miles an hour in her annual journey round the Sun. 

Q. Does the Earth move with the same velocity in all parts of her orbit? 

A. No. When she is in that part of her orbit nearest to the 
Sun, she moves with greater velocity than when she is at the point 
farthest from the Sun. 

Q. Why does she move with greater velocity when at her perihelion, that is, 
when in that part of her orbit nearest to the Sun ? 

A. Because the Sun exercises a greater attractive force upon 
the Earth when she is nearest to him. 

Q. If the Sun attracts the Earth more powerfully when in perihelion, why does 
she not fall to the Sun ? 

A. Because by the increased velocity of her motion, her centri- 
fugal force is also increased. 



SECTION III. 

" These as fhej cliange, Almiglity Fatlier, these 
Are but the varied God! The rolling year 
Is full of Thee." — Thomson. 
Q. What produces the change of seasons ? 

A. The change of seasons is produced by the Annual Motion 
of the Earth. 

Q. Upon what does the variety of seasons depend ? 

A. Upon the position of the Earth with respect to the Sun, and 
also upon the length of the day and night. 

Q. Are not the days and nights equal all over the Earth ? 

A. No ; they are of unequal lengths in different parts of the 
Earth. 

Q. Is there any part of the Earth where the days and nights are of equal 
lengths ? 

A. The days and nights are of equal lengths at the equator. 

Q. Why are the days and nights always of equal lengths at the Earth's 
equator ? 

A. Because the line which separates the illuminated from the 
dark hemisphere, must always divide the equator into tivo equal 
parts. 



MOTION OF THE EARTH. 



108 



Fig. 72. 




In Jig. 72, E E represents the position of the equator as 
regards the light and dark hemisphere, in both the summer 
and winter seasons of the other parts of the Earth. It will 
be observed that any place upon the equator would be car- 
ried by the Earth's diurnal rotation , half way through the 
circle in light and the other half in darkness. 



Q. How are the diflFerent portions of the year made evident to our senses ? 

A. By the changes from heat to cold, and the reverse. 

Q. What are the recurring hot and cold periods of the year called ? 

A. They are called summer and winter seasons. 
Q. Are the different seasons to be attributed to the Annual Motion of the 
Earth? 

A, Yes ; it is the cause of the various seasons. 

"The seasons, raonchs, and days, 

The short-lived offspring of revolving time ; 

By turns they die, by turns are born. 

No-w cheerful Spring the circle leads 

And strews with flowers the smiling meads ; 

Gay Summer next, whom russet robes adorn, 

And waving fields of yellow corn ; 

Then Autumn, who with lavish stores 

The lap of Nature spreads ; 

Decrepit Winter, laggard in the dance, 

(Like feeble age oppressed with pain,) 

A heavy season does maintain, 

With driving snows and winds and i-ain, 

Till Spring recruited to advance, 

The varied year rolls round again." — Hughes. 

Q. Why is one part of the year warmer than another ? 

A. Because all places on the Earth's surface are sometimes 

turned more towards, and at other times turned more awat/ from, 

the Sun's direct rays, 

Q. Why are all places on the Earth's surface sometimes turned more towards, 
and sometimes more away from, the Sun ? 

A. Because the Earth revolves round the Sun with the axis of 
its rotation held sideways, or inclined, instead of being perpen- 
dicular. 

It will be seen by the following figure that the north pole of the Earth is turned away 
from the Sun in December, and consequently the greater part of the northern hemi- 
sphere is in darkness, the shortest day being the 21st of that month. As the duration 
of sunlight is so short, and as the Sun's rays fall obliquely on the northern latitudes, it 
is winter. As the Earth moves in her orbit her north pole is more and more turned to 
the Sun, till about the 21st of March, when it will be seen that both poles are enlight- 



104 



BOUYIER S FAMILIAR ASTRONOMY. 



ened, the Sun being vertical at the equator. At this time the days and nights are 
equal, and spring commences. On the 21st of June the north pole is turned as much 
towards the Sun as the south pole was turned from it in December. At this period it 
is summer to the inhabitants of the northern hemisphere, and winter to those of the 
■southern. About the 21st of September the Sun is vertical again at the equator, and 
the season of autumn commences. 




Q. How mucli is the axis of the Earth inclined to the plane of her orbit ? 

A. The axis of the Earth is inclined twenty-three degrees 
twenty-eight minutes to the plane of her orbit. 

Q. What is the Earth's orbit called ? 

A. It is called the ecliptic. 

Q. What would be the effect if the axis of the Earth were not thus inclined 
to the plane of her orbit ? 

A. That part of the Earth called the torrid zone would be 
scorched, while from forty or fifty degrees on each side of the 
equator to the poles an unceasing winter would reign. 

"He bid Ms angels tiira askance 
Tbe poles of Earch twice ten degrees and more 
From tlie Sun's axis." — Milton. 



Q. Does EVERY portion of the Earth's surface enjoy its summer at the same 
time? 

A. No ; all that portion which is nearest to that pole which is 
turned towards the Sun is in the enjoyment of summer, while 
that portion in the vicinity to that pole turned away from the 
Sun is in winter. 

Q. Why does that part of the Earth nearest to the pole which is turned towards 
the Sun experience the warmth of summer ? 

A. Because the Sun's rays fall more directly upon that portion 



MOTION OF THE EARTH. 



m 



of the Earth's surface, and consequently exercise a greater heatin< 
power. 

Fig. 74. 




In the figure, the rays of the Sun are represented as proceeding from S ; they fall per- 
pendicularly upon E, the part of the Earth turned most towards the Sun, but obliquely 
upon R, the illuminated part of the Earth most remote from it. 

Q. Why do the Sun's rays heat a surface upon which they fall, more, when 
they strike it perpendicularly, than when they strike it obliquely ? 

A. Because when they fall perpendicularly, they are spread 
over a comparatively small space ; but when they fall obliquely, 
they are spread over a comparatively larger one. 

. Fig. 75. 




In the figure, let S S represent the space on which a given number of rays fall per- 
pendicularly j then S T would represent the space upon which the same number of rays 
would fall obliquely. But as S T is longer than S S, the same number of rays would 
be spread over a larger space in one case than in the other. 

Q. Is the perpendicular manner in which the rays of the Sun fall on the 
Earth's surface in summer the only cause of the heat of the season ? 

A. It is not; for in summer the days are always longer 
than the nights, and in winter they are shorter ; consequently, 
in summer we have a longer duration of sunlight than in 
winter. 

Q. Does the Earth absorb the heat of the Sun ? 

A. As long as the Earth's surface is exposed to the Suns rays, 
it absorbs heat, just as a body becomes heated when held before a 
blazing fire. 

Q. What is the effect when turned away from the Sun's rays ? 

A. As long as the Earth is turned away from the Sun's 
rays, it continues to radiate the heat which it had previously 
received ; consequently, the Earth's surface becomes heated 
during the long days of summer, and cooled during the long 
nights of winter. 



106 bouvier's familiar astronomy. 

"What prodigies can Power Divine perform 
More grand than it produces year by year ? 
And all in sight of inattentive man ! 
Pamiliar -with the effect, -we slight the cause. 
And in the constancy of Nature's course. 
The regular return of genial months. 
And renovation of a faded world. 
See naught to wonder at." — Cowper. 

Q. On what part of the Earth do the Sun's rays fall perpendicularly ? 

A. The Sun's rays can only fall perpendicularly upon the 
Earth's surface between the tropics. 

Q. Why are the Sun's rays perpendicular only to those places situated within 
the tropics ? 

A. Because the axis of the Earth being inclined only twenty- 
three degrees twenty-eight minutes, the Sun is vertical only to 
places situated within that distance of the equator, the boundaries 
of which space are the tropics of Cancer and Capricorn. 

The Sun is always vertical to some place within the tropics, and to no place heyond them. 
Q. Why are the days longer than the nights to all places where it is summer ? 

A. Because the pole nearest to those places is turned quite 
into the hemisphere of sunlight ; and consequently, all places which 
are not 90° (or a quarter of the Earth's circumference) from the 
enlightened pole, are more than half the time of the Earth's rota- 
tion in sunlight, and less than half that time in darkness. 



In the figure, let a represent the situation of Newfoundland 
upon the Earth's surface in the summer season, with n the 
northern extremity of the Earth's axis, turned quite into the 
hemisphere of sunlight; and let the line a h represent the 
parallel of latitude on which Newfoundland is situated. It 
will be observed that more than half that circle of diurnal 
motion lies in sunlight, and less than half of it in darkness. 



Q. When it is summer to the inhabitants of the southern hemisphere, what 
SEASON is it in the northern hemisphere ? 

A. To the inhabitants of the northern hemisphere it is winter 
when it is summer in the southern hemisphere, and vice versd. 

Q. Why are the nights of winter longer than the days ? 

A. Because the pole nearest to us is then turned quite into 
darkness; and, consequently, all places which are not 90° (that 
is, a quarter of the Earth's circumference) from that pole, are 
more than half the time of the Earth's rotation (that is, more 
than twelve hours) in darkness, and less than half the twenty-four 
hours in sunlight. 




MOTION OF THE EARTH. 



107 




Let a {jig. 1*1) represent the position of Newfoundland 
upon the Earth's surface in the lointer season. As the 
northern extremity of the Earth's axis is not illuminated by 
the Sun, those places in high northern latitudes must have 
but little daylight in the twenty-four hours. It will be seen 
that the parallel a 6 is much more than half in darkness ; 
consequently, at Newfoundland, the days in winter must be 
much shorter than the nights. 



Q. Does the length of the days at the poles vary very much ? 

A. During the summer, in the northern hemisphere, places 
near the north pole are in continual sunlight— the sun never sets 
to them ; while during that time places near the south pole never 
see the sun. And when it is summer in the southern hemisphere, 
and the sun shines on the south pole without setting, the north 
pole is entirely deprived of his light. 

Q. Is there any part of the year when the days and nights are of equal 
LENGTH all over the Earth ? 

A. Yes ; on the 21st of March and 21st of September the days 
and nights are of equal length. 

Q. Why are the days and nights of equal length all over the earth in March 
and September ? 

A. Because both poles are on the margin which separates the 
light from the dark hemisphere ; and consequently every place is 
carried by the Earth's rotation through a circle, which lies half 
in the light and half in the dark hemisphere. 

Q. How long is the longest day at the poles ? 

A. At the poles there is but one day and one night in the 
year ; for the Sun shines for six months together on one pole, 
and the other six months on the other pole. 

Q. Is the Earth's orbit circular ? 

A. No ; it is slightly elliptical. 

Q. Are we nearer to the Sun in summer than in winter ? 

A. No ; we are nearer to the Sun in December than we are in 

June. 

Q. How is it KNOWN that we are farther from the Sun in June than in De- 
cember ? 

A. It is known — 1. Because our summer, or the time between 
the vernal and autumnal equinoxes, is nearly eight dags longer 
than our winter, or the time between the autumnal and vernal 
equinoxes. 

2. The Sun's apparent diameter is greater in our whiter than 
in our summer ; and as the apparent diameter of objects increases 
as the distance is diminished, it follows that the Sun is nearer to 
us in our winter than in our summer. {See Note 25.) 



108 bouvier's familiar astronomy. 

Q. Has the elliptic form of the Earth's orbit any influence in producing the 
variation of temperature corresponding to the difference of seasons ? 

A. The ellipticity of the Earth's orbit has but trifling influence 
on the difference of seasons. 

Q. Is HEAT, like light, equally dispersed from the Sun in all directions ? 

A. It is. 

Q. Where is the perihelion point of the Earth's elliptical orbit situated ? 

A, Nearly at the place of the winter solstice. 

Thus the Earth is in perihelion, or that point of her orbit nearest the Sun, at the 
winter solstice, or about the 21st of December ; at which time it is midwinter in the 
northern hemisphere, and summer in the southern. 

Q. Then, as the orbit of the Earth is elliptical, is there not a greater degree 
of heat experienced in the southern hemisphere when we are in perihelion, or 
nearest to the Sun, than in the northern hemisphere when we are in aphelion ? 

A. No ; an equilibrium of heat is generally maintained in both 
hemispheres. 

Q. How is this equilibrium of heat maintained ? 

A. The Earth moves with greater rapidity in that part of its 
orbit nearest to the Sun than in that part more remote from it. 

Fig. 78. 

Let S represent the Sun's place in one of the foci 
of the Earth's elliptical orbit, ATP—. A is the 
aphelion, or farthest point from the Sun, and P the 
perihelion, or nearest point to it. As the planet 
moves from A to P its velocity will always be in- 
creasing, because it is drawing nearer and nearer to 
tbe Sun. But from P to A it will continually be de- 
creasing in velocity; therefore it moves through 
one-half of its orbit in nearly the same time in 
which it revolves through the other half. 




CHAPTER VIII. 



"WTien yonder spheres sublime 
Pealed their first note to sound the inarch of Time." 

Q. What is time ? 

A. Time is a degree of duration marked by periods or mea- 
sures, chiefly established by motions of the heavenly bodies ; as a 
year, a month, a day, &c. 

Q. What NATURAL occurrence is taken as the most convenient measure for 
estimating the lapse of time ? 

A. The Earth's rotation on its axis. 

Q. Why is the Earth's rotation on its axis the best available unit or standard 
for the measurement of time? 

A. Because the Earth's rotation is the most equable and uni- 



TIME. 109 

form motion in nature; every revolution on its axis being per- 
formed in exactly the same space of time. 

Q. How is time divided ? 

A. Into years, months, weeks, days, hours, minutes, and se- 
:Jonds. 

Q. How many kinds of time are there ? 

A. There are two kinds of time noted by astronomers : viz. 
sidereal and solar time. 

Q. What is sidereal time ? 

A. Time measured by observing the apparent motions of the 
stars. 

Q. What is solar time ? 

A. Time measured by observmg the apparent motion of the 
Sun. 

Q. How is it KNOWN when the Earth has completed one revolution upon its 
axis? 

A. It is noticed that the spot on which the observer stands 
has come once again between the Earth's axis and some well- 
known star. 

Q. What is this unit of time called in which the Earth completes one revolu- 
tion upon its axis ; that is, if a well-known star be on the meridian, till it comes on 
the meridian again ? 

A. It is called a sidereal 



The interval between two successive passages of a star over the meridian of any place 
is called a sidereal day. 

Q. What is that unit of time called in which the Earth, by revolving 
on its axis, brings the Sun from the meridian of an observer to that meridian 
again ? 

A. It is called a natural or solar day. 

Q. What is the length of a sidereal day ? 

A. The sidereal day consists of twenty-three hours, fifty-six 
minutes, and four seconds. 

Q. What is the length of a natural or solar day ? 

A. The solar day consists of twenty -four hours. 

Q. What is the reason of this difference in the length of a sidereal and a 
solar day ? 

A. It is caused by the fact that any given spot on the Earth's 
surface does not arrive between the Earth's axis and the Sun a 
second time, so soon as it is again between the Earth's axis and 
a fixed star. 

Q. Why is it that a given spot on the Earth's surface does not arrive a second 
time between the earth's axis and the Sun, so soon as it does a second time be- 
tween the Earth's axis and a star ? 

A. This is because the Earth is moving round tlie Sun, and is 
at the same time revolving on its axis. 



110 



BOUVIER S FAMILIAR ASTRONOMY. 



Fig. 79. 
I 




Let S {jig. 79) represent the Sun, and 1 2 the 
the distance through which the Earth has moved 
in its orbit, when it has completed one revolution 
on its axis, in the direction w n e d, known by 
some given point o, having again come under 
the perpendicular line I o, representing the direc- 
tion in which some star is viewed. Let it be re- 
membered that the fixed stars are at such im- 
mensely great distances from us, that the two 
lines, lo, I o, will make no perceptible angle. It 
will l3e seen that in the position 2 the spot o is no 
longer opposite to both the Sun and the star, as 
it was in the position L The spot o will then 
have to turn on till it comes to d before it is again 
opposite to the Sun. The time which it takes to 
move from the line I o io d is the difference be- 
tween a solar and a sidereal day. 



"Watch, -with, nice eye the Earth's diurnal ■way, 
Marking her solar and sidereal dat ; 
Her slo-w nutation, and her varying clime. 
And trace -with, mimic art the m^rch. of Time." — Botanic Garden. 

Q. Is the solar day, like the sidereal day, always of one length ? 

A, It is not. Some solar days are much longer than others, 
when measured by the apparent return of any given spot on the 
Earth's surface to a position between the Earth's axis and the 
Sun's centre. 

Q. Why are some solar days longer than others ? 

A, Because the Earth moves faster in those parts of its orbit 
nearer the Sun, than in those the farthest from it. 

Q. Why is the solar day taken for the common reckoning of time, since it is 
so much less constant as a measure than the sidereal day ? 

A. Because its middle is marked out conveniently by the mid- 
dle of daylight. It is obviously convenient to have days and 
nights falling regularly amid the ordinary occupations of life. 
(See Note 26.) 

Q. Do the common clocks that keep solar time follow the Sun's irregularities ? 

A. No ; common clocks are made to go, not by apjjarent solar 
time, but by what is called mean solar time. 

Clocks regulated by the stars give sidereal time, and are used in astronomical obser- 
vations ; one revolution of the clock through twenty-four hours represents one apparent 
revolution of the starry sphere. When the clock shows OA. Om. Os. the first point of Aries 
is or. the meridian: and this being the point from which right ascension is reckoned, 
the time by the sidereal clock is always the right ascension of such stars as are passing 



TIME. 



Ill 



the meridian above the pole at the moment. Wherefore, by noting the time when a star 
IS in this position, we have its distance from the first point of Aries. 
Q. What is meant by apparent time ? 

A. Apparent time is measured by the apparent motion of the 
Sun in the heavens, or by a good sun-dial. 

Q. What is mean time ? 

^ A. Mean time is measured by a well-regulated clock, that goes 
without variation, measuring twenty-four hours from noon to noon. 
Q. How is mean time reckoned ? 
A, By the average length of all the solar days throughout the 

y^^^- Fig. 80. 




This IS the period of time called a civil day, which consists of twenty-four hours, be- 
ginning at midnight. An exact knowledge of time is of the highest importance in navi- 
gation The sailor endeavors to find, before he sails, how much his chronometers mly 
vary from the true time, so as to be enabled to correct the error when at sea For this 
purpose many seaports have a signal to indicate the moment of one o'clock " A ball is 



112 BOUVIER'S FAMILIAR ASTRONOMY. 

attached to a mast or tower in some open situation, and at five minutes before one, tho 
ball rises to the top ; the moment it is one o'clock, the ball leaves the top of the mast. 
The figure represents the ball on the top of the observatory at Greenwich. The error 
in letting off this ball is less than two-tenths of a second. 

Q. What is the equation of time ? 

A. It is the difference between mean and apparent time ; that 
is, the dfference of time as shown by a well-regulated clock and 
a true sun-dial. 

As apparent time is sometimes greater and sometimes less than mean time, it follows 
that in some parts of the year they must coincide. This happens at present about the 
15th of April, the 15th of June, the 1st of September, and the 22d of December. 
Q. What is a year ? 

A. The period that is marked by the Sun's being vertical a 
second time at one and the same tropic. 

Q. What is the year called that is measured by the Earth moving so as to bring 
the Sun vertical a second time to the same tropic ? 

A. It is called the tropical year. 

Q. Has the Earth revolved completely round the Sun in a tropical year ? 

A. No; the Earth requires twenty minutes and twenty seconds 
longer to make one complete revolution round the Sun than it 
does to cause the Sun to appear vertical a second time at either 
of the tropics. 

Q. How is it KNOWN when the Earth has accomplished one complete revolution 
round the Sun ? 

A. The Earth is known to have completed one revolution round 
the Sun, when it has again arrived between the Suns centre and 
some marked star. 

Q. How is it known when the tropical year is completed ? 

A. The tropical year is completed when the Sun has returned 
a second time to the same equinoxes or solstices; which apparent 
revolution of the Sun is caused by the real motion of the Earth 
round that luminary. 

Q. What is the period called in which the Earth arrives again to a station 
between the Sun and the same star ? 
A. It is called a sidereal year. 
\ \ Q. How LONG is the sidereal year ? 

A. The sidereal year is equal to 365 days, 6 hours, 9 minutes, 
and 9 seconds. 

Q. What is the length of the tropical year ? 

A. The tropical year is equal to 365 days, 5 hours, 48 minutes, 
and 49 seconds. 

Q. What causes this difference between the sidereal and tropical year? 

A. It is caused by the fact that the axis round which the 
Earth rotates does not always continue in the same precise direc- 
tion in space, but vacillates a little as it revolves. 

Q. What is the meaning of the statement, that a tropical year consists of 365 
days, 5 hours, 48 minutes, and 40 seconds? 

A. It is meant that in that time the Earth has revolved round 



TIME. , 113 

its 02vn axis 365 times, and as near a quarter of another time as 
5 hours, 48 minutes, and 49 seconds is to 6 complete hours. 

Q. Since the Earth does not turn on its axis any exact number of times in a 
year, how is it that every year begins with the commencement of a day? 

A. During three years the superfluous hours and minutes by 
which 365 days are exceeded are disregarded ; but on every fourth 
year they are put together and reckoned as an additional day, 
properly belonging to that year. 

Q. How many days does each fourth year consequently contain ? 

A. Each fourth year contains three hundred and sixty-six days. 

Q. What is the long year called, which contains 366 days ? 

A. It is called leap year. 

Leap year is also called Bissextile. The Romans inserted the intercalary day between 
the 23d and 24th of February ; the 23d of February in their calendar was called sexto 
ealendas 3fartii, the sixth of the calends of March ; and the intercalated day was called 
his sexto ealendas Martii, the second sixth of the calends of March; from whence our 
intercalated year is called Bissextile, or leap year. 

Q. To what PART of the leap year is the 366th day added ? 

A. It is placed at the end of the shortest month ; that is, at the 
end of February ; consequently, every leap year has a 29th day 
of February. {See Note 27.) 

Q. Is there any rule whereby we may know what year is leap year? 

A. It is known by dividing the number of the year by four^ and 
if there be no remainder, it is leap year ; the year 1870, when 
divided by 4, leaves a remainder of two; therefore, it must be the 
second year after leap year. 

"Divide by 4, whab's left shall be 
For leap year, ; for past, 1. 2, 3." 

Q. Does the insertion of a leap year after every third common year, make the 
reckoning by solar days and tropical years exactly correspond ? 

A. No, not exactly. It would do so if the tropical year con- 
sisted of 365 days and 6 hoxvc^ precisely ; but as it wants 11 minutes 
and 11 seconds of completing the 6 hours, the insertion of one 
day in every four years does not make the reckoning exactly cor- 
respond. 

Q. How is this error remedied ? 

A. By inserting a day in every year divisible by four and not 
divisible by a hundred, and also by inserting a day in every year 
divisible by 400. 

Q. What is the consequence of this reckoning not corresponding exactly? 

A. The consequence is, that there is an error of one day too 
many in every three thousand six hundred years. 

Q. How is this error corrected ? 

A. To correct this error, a day is always left out of those leap 
years whose remainder is exactly divisible hj four thousand zvith- 
out a remainder. 



—■I 



114 BOUVIfm'S FAMILIAR ASTRONOMY. 

Q. Does this correction remedy the error? 

A. This error does not amount to an entire day until after the 
lapse of one hundred thousand years. 

Q. When it is 12 o'clock or noon at Washington, is it noon at all places in 
the United States ? 

A. No ; it is noon to such places only as are on the same meri- 
dian as Washington. 

Q. Suppose, then, an eclipse, or other astronomical observation, to be made at 
Washington and at Philadelphia by different persons, and the time at both places 
noted, would their time be the same ? 

A. No ; the time would be a little later at Philadelphia, be- 
cause it is a little farther east. 
Q. What is meant by a cycle ? 

A. A periodical space of time, in which the same revolutions 
of the heavenly bodies return again to the same days of the 
week and month. 

Q. What are the principal cycles, and what is their use ? 

A. The principal cycles are the cycle of the Sun and the cycle 
of the Moon; they were invented for measuring the periodical 
motions of the heavenly bodies, and for obtaining a more exact 
computation of time. 

Q. What is meant by a cycle of the Sun ? 

A. A revolution or period of twenty-eight years ; at the expira- 
tion of which the days of the month return again to the same 
days of the week. 

Q. What is a cycle of the Moon ? 

A. A cycle of the Moon consists of a period of nineteen years ; 
after which the various aspects of the Moon are nearly the same 
as they were on the same days nineteen years before. {See Note 28.) 

This cycle was adopted on the 16th of July, b. c. 433, by Meton, an Athenian, and is 
also known by the name of the Metonic cycle. The period of seventy-six years, or four 
Metonic cycles, was invented by Callippus, and is hence called the Callippic period. 



CHAPTER IX. 



Q. What is the ecliptic ? 

A. The ecliptic is an imaginary great circle in the heavens, 
through which the Sun appears to move once a year, but which 
is really the path which the Earth describes around the Sun. 

Q. What is the equator ? 

A. The equator is an imaginary circle situated equidistant 
from the poles, and which divides the Earth into two hemispheres — 
the northern and the southern. 



THE ECLIPTIC AND ZODIAC. 



115 



Q. What is the equinoctial line ? 

A. The equinoctial is the plane of the equator extended to the 
heavens, and is sometimes called the celestial equator, 

Q. Is the ecliptic parallel to the equinoctial ? 

A, No ; it is inclined about 23° 28' to the equinoctial. 

Q. Where is the ecliptic to be found in the heavens ? 

A. Half 0^ the ecliptic line is north of the equinoctial, and 
half of it south of the equinoctial. 

Q. The equator of the heavens, or equinoctial, makes an angle with the ecliptic 
of 23° 28^ : what is this angle called ? 

A. This angle is called the obliquity of the ecliptic. 

Q. What are the equinoxes ? 

A, The two points where the celestial equator is intersected bj 
the ecliptic are called the equinoxes. 

Q. What are the solstices ? 

A. Those two points on the ecliptic where the Sun is at his 
greatest distance north or south of the equator. 

Q. What are the colures ? 

A. Two imaginary circles or meridians, one of which passes 
through the equinoctial points, and is called the equinoctial colure ; 
the other passes through the solstitial points, and is called the 
solstitial colure. {See Note 29.) 

Q. What is meant by heliocentric and geocentric place of a planet ? 

A. The heliocentric place is the point it occupies as seen from 
the Sun ; the geocentric place, the point it occupies as seen from 
the Earth. Pi^ si 




I 



In the figure, the heliocentric place of Venus is in Leo, whereas the geocentric place 
is in the beginning of Scorpio. The heliocentric place of Mars is in Taurus, and tha 
geocentric place in Aries. 



116 bouvier's familiar astronomy. 

Q. What is the zodiac ? 

A. An imaginary zone or belt extending around the heavens. 
It is sixteen degrees broad, through the middle of which is the 
ecliptic. 

Q. Wliat is the meaxixg of the word zodiac ? 

A. It is derived from a Greek word signifying an animal^ be- 
cause the ancients represented each sign of the zodiac by some 
animal. 

Q. How MANY SIGNS are there in the zodiac ? 

A. There are twelve signs in the zodiac, the signs and Latin 
names of which are as follows : — 



T Aries. 
b' Taurus. 
)X Gemini. 
® Cancer. 
Q. Leo. 
1f Yirgo. 



^ Libra. 
1U Scorpio. 
7(^ Sagittarius. 
^ Capricornus. 
— Aquarius. 
K Pisces. 



But in the following lines the English names are used, which will be more easily re- 
membered, as well as the order in which they stand : — • 

" The Ram, the Bull, the Heavenly Twins, 
And next the Crab the Lion shines, 

The Virgin aul the Scales; 
The Scorpion, Archer, and Sea Goat, 
The Man that holds the watsrinq pot, 
And Fish with glittering tails." 
Q. What is meant by a sign of the zodiac ? 

A. Every circle is divided into 360 degrees. Now, for conve- 
nience, the zodiac is divided into twelve equal parts, each of 
which, therefore, contains the tivelfth part of 360 degrees ; that 
is, thirty degrees. These twelve parts are denominated signs. 

Q. Is there any thing remarkable to be observed with regard to the zodiac ? 

A. The zone known as the zodiac is remarkable from its being 
the area or space within which the apparent motions of the Sun, 
Moon, and all the planets (except some of the asteroids) are 
confined. 

All the planets, (except some of the asteroids,) as well as the Sun and Moon, are to 
be found in this space called the zodiac, which extends eight degrees on each side of 
the ecliptic. 

Q. AVhy are the planets and Moon to be found in that place called the zodiac ? 
A. Because their orbits are not inclined more than eight de- 
grees to the ecliptic ; therefore the boundaries of the zodiac are 
the extreme limits of their motions. 

The Sun is always in the zodiac, because the ecliptic is his apparent path. 
Q. How is the zodiac marked on the artificial globe ? 

A. Each sign is marked by the picture of some animal, per- 
son, or figure, because the ancients imagined the stars were 
grouped so as to form resemblances to such objects. 



TIDES. 



117 



CHAPTER X. 

" TTie ebb of tides, and their mysterious flow, 
We, as art's eleraents, shall understand." — Detden. 

Q. Why does the Moon move round the Earth ? 

A. Because the Earth's mass attracts the Moon's mass ; the 
Moon is also under another influence, called the centrifugal force, 
which, combined with the Earth's attraction, retains it in its orbit. 

Q. Does the Sun's mass also influence the Moon's mass? 

A. Yes ; the Moon is attracted hy the Sun as well as by the 
Earth. 

Q. How is the Sun's attractive influence over the Moon made manifest? 

A. By certain perceptible irregularities of movement which 
the Moon suffers. 

Q. Does the Moon's mass attract the Earth's mass, as well as suffer its 
attraction ? 

A. It does; but as the Moon's mass is 80 times less than the 
Earth's mass, the Moon's attractive power is also 80 times less 
than that of the Earth. 

Q. Is the attraction of gravitation universal ? 

A. Yes ; the attraction of gravitation is universal, being pre 
sent in all material bodies. 

Every ponderable substance attracts every other ponderable substance in nature. 
Q. How is the attraction of the Moon's substance for the Earth's substance 
manifested ? 

A. By the swelling of the waters of the ocean up towards the 
Moon. 

"Attractive Power! whose mighty s-way 
The ocean's swelling waves obey ; 
And m.ounting upward, seem to raise 
A liquid altar to thy praise." 

Q. What is the difference between the movements, as a whole, of solid bodies 
and fluid bodies ? 

A. Solid bodies can only move in a mass; but fluid bodies can 
be made to surge hither and thither, independently of any motion 
in the entire mass. Fluids are distinguished from solids by this 
power which their particles possess of moving about freely among 
each other. 

Q. What is the swelling up of the water towards the Moon called ? 

A. It is called a tide or tidal wave. 

Q. Does the tidal wave rise highest on the part of the Earth's surface that is 
nearest to the Moon? 

A. Not exactly on that part. The tidal wave rises highest on 



118 bouvier's familiar astronomy. 

a portion of the Earth's surface that has been a little revioved 
from this position by the progress of the EartJis rotation. 

This is called the lagging of the tides, which is illustrated by the following figure. 

Fig. 82. 



Suppose that the Earth E is turning round in the direction haJsJ, {fig. 82,) the high- 
est point of the tidal wave, caused by the Moon's attraction, lies not at h, but at a ; and 
the opposite wave is in the corresponding position, as at I. 

Q. Why does not the apex of the tidal wave fall on the part of the Earth's sur- 
face that is jfEAEEST to the Moon ? 

A. Because the rising of the water is a gradual process, and re- 
quires time for its accomplishment after the influence which causes 
it has come into operation. 

Q. Why does not the water move instantaneously into the position which the 
Moon's attraction draws it ? 

A. The inertia of the particles of water, and their friction 
against each other, combine to prevent their moving instantly 
into the position which the Moon's attraction strives to place them. 

On account of the inertia of the waters, the tidal wave is not directly under the Moon, 
but under a meridian 30° eastward of it, in the hemisphere nearest the Moon, as well 
as on the opposite half of the Earth. On the west side of this great wave the tide is flow- 
ing; on the east it is ebbing ; and on the meridian, 90° distant, it is low water. 

Q. Does the swell of the tidal wave always hold the same position on the 
Earth's surface ? 

A. No; the tidal wave sweeps along over the surface of the 
ocean, and having passed round the Earth arrives again at the 
spot from which it started, after an interval of twenty-four hours 
and fifty minutes. 

Q. Why does the tidal wave of the ocean sweep bound the Earth's surface ? 

A. Because fresh portions of the surface of the ocean are con- 
tinually passed in succession before the moon, in consequence of 
the Earth's rotation on her axis. 

More properly, the Earth, rather than the wave, revolves. 



TIDES. 



119 



Q. But the Earth turns on its axis once in twenty-four hours : why, then, is the 
ewell of the tidal wave fifty minutes longer before it recurs at any given place ? 

A. Because during the twenty-four hours which the Earth has 
been turning on its axis, the Moon has advanced so far in her 
orbit, that the Earth must revolve fifty minutes longer before the 
same spot can be directly under her influence again. 

Q. If the Moon be observed to be over some fixed object, such as a chimney, 
on one night at a certain hour, will it be over the chimney on the next night at 

the SAME HOUR ? 

A. No ; it will be found to hold the same position about fifty 
minutes later on the following evening. 

Q. Why is the Moon later in arriving at the same position on the following 
evening ? 

A. Because when the Earth arrives into the same position, the 
Moon has moved in her orbit so far, that the Earth must rotate 
fifty minutes before it will bring the chimney into the same situa- 
tion with regard to the Moon. 

Fig. 83. 




In the figure let M represent the Moon, moving in her orbit in the direction of the 
arrows. On a given day the Moon is vertical to a spot on the Earth's surface marked A. 
After a lapse of twenty-four hours the point A will have returned to the same position, 
but the Moon will have moved in her orbit from one to two ; consequently, it would be 
vertical at a point on the Earth marked B. It will be seen that the observer at A will 
not see the Moon on his meridian until the Earth shall have revolved from A to B, 
which would require about fifty minutes. 

Q. Does only one tidal wave sweep over the ocean's surface, in each interval 
of twenty-four hours and fifty minutes ? 

A. No ; two high tides occur in that interval, each following 
the other at a period of twelve hours and twenty-five minutes. 

"Alternate tides in sacred order run." — Blacemore. 
Q. Why arc there two tidal waves in twenty-four hours and fifty minutes ? 
A, Because the water is attracted on the side of the Earth yiext 
to the Moon ; and it is left on the other side of the Earth, because 



,li 



120 bouvier's familiar astronomy. 

the Moon attracts the Earth more than the waters which cover 
the hemisphere opposite to the Moon. 

Q. Why is there a second high tide on that part of the Earth's surface which 
is farthest from the Moon ? 

A. Because the water there is less influenced by the Moon's 
attraction than the solid nucleus of the Earth. As the water 
<ioes not suffer so much attraction as the Earth, it is left behind. 
{See Note 30.) 

"The Moon, the governess of floods." — MrosuMUEK Night's Dkeam. 

Q. Does the tidal wave of the ocean always rise to the same height ? 

A. It sometimes rises considerably higher than it does at others. 

Q. Why does the tidal wave sometimes rise higher than it does at others ? 

A, Because another force sometimes aids the Moon in heaping 
up the water, but at other times opposes its influence. 

Q. "What is that other force that sometimes aids and sometimes opposes the 
Moon's attractive power ? 

A. The attraction of the Sun; for since the Sun attracts the 
Earth, it must also attract the superficial water of the Earth. 

Properly speaking, there are two series of tidal waves travelling over the surface of 
the ocean; the one series lunar, the other solar; the first depending on the position of 
the Moon, the last on that of the Sun. 

Q. Why is the Sun's attractive force less than that of the Moon? 

A. Because the Sun is so mnah. farther horn the Earth than 
the Moon. {See Note 31.) 

Q. How MUCH less influence does the Sun exert over the waters of the ocean 
than the Moon ? 

A. The Sun's influence is less than the Moon's by more than 
one-half. The Sun's attraction raises the level of the water of 
the ocean about two inches for every five inches of elevation 
effected by the Moon. 

Fig. 84. 





Let S be the Sun, and M the Moon in her orbit M A B. When the Moon is between 
the Earth and the Sun, as at M, the attractive force of the Sun is combined with that 
of the Moon to raise the waters of the Earth at a ; and by attracting the Earth more 
powerfully than the waters on the opposite hemisphere at c, they are left behind, and 
consequently are raised above the general level ; while at h and d it is low tide. 



TIDES. 121 

Q. When does the solar influence add its effect to the lunar tidal wave, and 
AUGMENT its efi"ect ? 

A, When the Sun is placed on the same side of the Earth with 
the Moon, and when it is placed on exactly the opposite side to 
the Moon ; that is, at the periods of new anadfuU moon. 

Q. Why does the solar influence add its efi'ect to the lunar tidal wave when 
the Sun and Moon are both placed on the same side of the Earth ? 

A. Because then the Sun and Moon both attract in the same 
direction. This is evidently the case at the time of new moon. 

Q. Why does the solar influence also add its efi'ect to the lunar tidal wave when 
the Sun and Moon are on opposite sides of the Earth ? 

A. Because then the secondare/ solar wave coincides in position 
with the primary lunar wave. This occurs at the time of full 
moon. 

Fig. 85. 




It will be seen on reference to^gr. 85, that the attractive influence of the Moon M, is 
in a line with that of the Sun S, thereby creating high tides at a and c, and low tides 
at h and d. 

Q. What are the tides called that rise the highest because the attractive influ- 
ences of the Sun and Moon combine to act together? 

A. They are called spring tides. 

Q. When do spring tides happen ? 

A. Spring tides happen soon after new and full moons. 

Q. When does the Sun attract the water of the ocean in such a way that the 
height of the lunar wave is reduced by its influence ? 

A. At the first and third quarters of the lunation. 

Q. Why does the solar influence diminish the efi'ect of the lunar influence at 
the first and third quarters of the lunation ? 

A. Because the Sun's attraction then draws the water of the 

ocean towards a position situated midway between the spots on 

which the crests of the two lunar waves occur. 

Q. What are the tidal waves of inferior height called, which occur about 
the times of the Moon's quarters? 

A. They are called neap tides. 

Q. What is the cause of spring tides ? 

A. Spring tides are the result of the addition of the Sun's in- 
fluen'^e to the Moon's influence. 



I 



i ! 



122 bouvier's familiar astronomy. 

Fig. 86. 




•« =f s 



It will be seen by reference to jig. 86, that when the Sun and Moon are 90° apart, 
that is, when the Moon is in quadrature, as at M, the solar and lunar influences tend to 
neutralize each other. The solar waves at a and c tend to neutralize the lunar waves 
at b and d. 

Q. What is the cause of neap tides ? ' 

A. Neap tides are the result of \he predominance of the Moons 
influence over the Sun's influence. 

Q. What is the difference of the height to which spring tides and neap 
tides rise ? 

A. Average spring tides rise more than twice as much as ave- 
rage neap tides. 

If the influence of the Moon be represented by 5, then the influence of the Sun must 
be represented by 2 ; but as the spring tides are caused by the combined influences of 
the Sun and Moon, their average height may be expressed by 7, which is formed by 
adding 5 and 2 together ; while the average height of the neap tides may be expressed 
by 3, which is formed by taking 2 from 5 ; because they are caused by the predominance 
of the lunar over the solar power. 

Q. Do the spring tides of the ocean always rise to the same height ? 

A. No ; spring tides generally rise as much more than neap 
tides as 7 is greater than 3 ; but sometimes they rise as much 
more as 10 is greater than 3. 

Q. What is the difference between the rise of the highest and the lowest 
tidal wave ? 

A. The highest rise of the tidal wave is rather more than three 
times the lowest rise. 

Q. Why do the spring tides rise to different extents at diflferent times ? 

A. Because the Sun and Moon are at different distances from 
the Earth at different times. 

Q. Why do the Sun and Moon exert different influences at different 
distances ? 

A. As the power of attraction acts with diminishing energy at 
increasing distances^ consequently both Sun and Moon must influ- 
ence the waters of the Earth less when they are more distant 
from them. 

The power of the Sun varies from this cause by nearly one-tenth, and that of the Moon 
by nearly one-half. 



ECLIPSES. 123 

Q. Does any given tidal wave rise equally throughout the whole euRf ace of 
the ocean ? 

A. No ; each tidal wave rises higher in that part of the ocean 
that is most immediately under the Moon and Sun. [See Note 32.) 

Q. Are lakes and small bodies of water subject to tides? 

A. No; the disturbing action of the Sun and Moon can only 
become sensible on large bodies of water. 

Q. What large bodies of water are the principal sources of our tides ? 

A. The oceans, especially the Pacific. 

The tide raised in the Pacific is transmitted to the Atlantic Ocean, and moves in a 
northerly direction along the coasts of Africa and Europe. It is, however, modified 
by the tide raised in the Atlantic. Sometimes the tides of these two great oceans are 
united, and at other times are in opposition to each other, so that the tides rise higher 
or lower in proportion to their combination or difi"erence. 

Q. Do tides rise to the same height in the open sea that they do in channels? 

A. They do not. They rise to a much greater height in nar- 
row channels than in the open sea, on account of the obstructions 
they meet with; 

At St. Malo, on the coast of France, the tides rise as high as fifty feet, because the 
water is more confined in the British Channel than in the wide ocean. On the shores 
of some of the South Sea islands, near the centre of the Pacific, the tides do not rise 
above one or two feet. In some parts of the Bay of Fundy the tides rise to the height 
of 120 feet. 

Q. Have the winds any influence on the tides ? 

A. They have; as they conspire with or oppose the tides, they 
serve to raise or depress the tidal wave. 

Although the wind has a powerful effect upon the surface of the ocean, its disturbing 
influence does not extend more than a hundred feet below. 

Q. What is the rising and falling of the tides called? 

A. The rising of the water is called ihQ flow, the falling, the 
ehh, of the tide. 

Q. How long is the tide rising before it reaches its maximum ? 
A. About six hours; and after remaining a quarter of an hour 
stationary, it is about six hours falling. 
Q. Of what USE are the tides ? 

A, The waters of the ocean are prevented from becoming stag- 
nant; and the currents thus created equalize its temperature. 



CHAPTER XI. 

Q. What is an eclipse ? 

A. The interposition of an opaque celestial body, or its shadow, 
between another celestial body and the observer. 

Q. What PRODUCES eclipses? 

A. The shadoiv of an opaque body thrown on another by a 
luminary much larger than either. 



124 



BOUVIER S FAMILIAR ASTRONOMY. 



Q. What is the form of the shadow ? 
A. It is conical. 

Q. Why is it conical ? 

A. Because all the heavenly bodies known to us are spherical^ 
and the shadow of a spherical body thrown by a luminary larger 
than itself must necessarily be in the form of a cone. 




If the luminary were of the same size as the intervening body, the shadow would be 
as represented in Jig. 1 of the above diagram. If, on the contrary, it were smaller than 
the body, the shadow would diverge, as in fig. 2. But the shadows are known to bo 
conical ; therefore, the luminary must be larger than the body, as shown in fig. 3. 
Q. In what direction is the shadow of a body cast ? 

A. The shadow of a body always falls towards a point directly 
oj>posite to the luminary. 

Q. What IS a shadow ? 

A, A privation of light. 

Q. What is this entire privation of light, or conical shadow, called? 

A. It is called the umbra. 

Q. Is there any other kind of shadow to be perceived in an eclipse ? 

A. There is Sb faint or partial shade observed between the per- 
fect shadow and the full light. 

Q. What is this partial shade called ? 

A. It is called the penumbra. 




125 



By referring to fig. 88, a dark conical shadow, marked U, will be seen in the direc- 
tion opposite to the Sun: this is the umbra. The fainter shadow, marked P P, diverging 
from the Earth, is called the penumbra. They are both conical, that of the umbra 
having for its base the diameter of the Earth, its apex being the point a • the penumbra 
is in the form of a truncated cone c d ef, the base e/ extending out into space. 

Q. How MANY KINDS of eclipses are there ? 

A. Two ; solar and lunar eclipses. 

Q. When do eclipses occur ? 

A. Eclipses can only occur when the Moon is in or near her 
nodes. 

Q. What are the Moon's nodes ? 

A. Those points of her orbit which intersect or cross the orbit 
of the Earth. 

Q. What is meant by the Moon being in or near her nodes ? 

A. She is then in or near that part of her orbit which ci^osses 
the eeliptic. 

Q. Is there any heavenly body nearer to the Earth than the Moon ? 

A. No ; the Moon is the Earth's nearest neighbor in space. 

Q. How does the Moon move with regard to the Earth ? 

A. The Moon moves round the Earth as its immediate and 
constant attendant. 

Q. How is it KNOWN that the Moon moves round the Earth ? 

A. It is seen to move around it among the stars. If the Moon 
be observed from night to night, it will be found to be constantly 
leaving the stars to the westward. 



SECTION I. 

^olar Eclipses. 

Q. Does the Moon ever pass apparently near to the Sun as she revolves round 
the Earth ? 

A. She does ; and under certain circumstances she is seen to 
move directly between us and the Sun. 

Q. What is a passage of the Moon between us and the Sun called ? 
A, It is called a solar eclipse. 



126 



BOUVIER S FAMILIAR ASTRONOMY. 



Q. Does the Moon always hide the Sun from sight when passing through that 
portion of its orbit that lies nearest to the great luminary ? 

A. No ; it often passes through that portion of its orbit many 
times in succession withotit concealing the Sun. 

Q. Why does not the Moon always conceal the Sun when moving through this 
portion of its orbit ? 

A. Because it then moves across the heavens, sometimes above, 
and sometimes below, the position the Sun occupies. 

Q. Why does the Moon sometimes move across the heavens above the Sun's 
position, and sometimes below it ? 

A. Because the orbit in which the Moon revolves is not in the 
same plane with the Earth's orbit. 

Q. Is the Moon's orbit inclined to that of the Earth ? 

A. The Moon's orbit is inclined to the Earth's orbit by rather 
more than 5°. {See Note 33.) 

Q. Why is it, then, that the Moon does sometimes pass between the Earth and 
the Sun ? 

A. The direction in which the plane of the Moon's orbit lies is 
not a constant or invariable one : it is continually changing. 

Q. Does it change its inclination to the plane of the Earth's orbit ? 

A. It very nearly always preserves the same inclination to the 
plane of the Earth's orbit while it is changing its direction. 

If the pupil will drive a nail part way into the centre of a circular piece of wood or 
pasteboard, and then place it upon the table, so that the circular piece of wood shall 
rest upon its own edge and the head of the projecting nail, a slight force will make it 
move round, still resting partly on the head of the nail, and thus forming an inclination 
to the flat surface or plane of the table. As it moves, the direction in which the sur- 
face of the piece of wood lies will continually change, although the angle between it 
and the table will always remain the same. The plane of the Moon's orbit moves in 
this way ; for, although continually changing, it always retains the same angle with the 
orbit of the Earth. 

Q. What are those points of the Moon's orbit called which lie in the same 
plane with the Earth's orbit ? 

A. The points where the lunar orbit crosses the plane of the 
Earth's orbit are called the nodes of the Moon's path. 

Fig. 89. 





Let E S {jig. 89) represent the plane in which the Earth moves about the Sun, and 
let n anp represent the Moon's orbit; then the points n n of the Moon's orbit, which lie 
in the same plane with the Earth's orbit, are the nodes, (from a Latin word signifying 
knots, because the terrestrial and lunar orbits are, as it were, knotted together there=) 
The part j9 of the Moon's orbit nearest to the Earth is called, in astronomical language, 



ECLIPSES. 



127 



its perigee ; a, tlie part of the Moon's orbit farthest from the Earth, is called its apogee; 
and a h, the line connecting the nearest point with the farthest point, is called the line 
of the apsides. 

On account of the perturbations to which the Moon's motions are subject, the form 
and direction of the lunar orbit are constantly changing, and fresh points of it become 
nodal in succession. Its plane rolls round as described above ; and in addition to this, 
the line of apsides moves round in the plane of the orbit, as the two opposite spokes of 
a revolving wheel. The Moon revolves in the ellipse h n a n, it will be remembered, in 
a little more than 27 days. The plane of the orbit contained within the ellipse Tin an 
revolves, as the circular piece of wood is described to do, in a little more than 18 years. 
{See Note 34.) 

Q. What causes a solar eclipse ? 

A. A solar eclipse is caused by the interposition of the body 
of the Moon between the Sun and the Uarth. ^ 



"As when the Sun, new risen, 

Looks through the horizontal, misty air 
Shorn of his beams, or from, behind the Moon 
In dim eclipse, disastrous twilight sheds 
On half the nations, and with fear of change 
Perplexes monarchs." — Paradise Lost. 

Q. When can a solar eclipse take place ? 

A. A solar eclipse can only take place when one of the nodes 
of the Moon's orbit is nearly in a line between the Earth and 
the Sun at the time of new moon. 

Q. Is not the Moon of smaller size than the Sun ? 

A, Yes ; the Moon's sphere is four hundred times less in dia- 
meter than that of the Sun. 

Q. How is it, then, that the large Sun can be hidden behind the small 
Moon ? 

A. Although the Moon is 400 times smaller than the Sun, it is 
also 400 times nearer to us. 

Fig. 90. 




As the Sun is 400 times larger than the Moon, and also 400 times more distant, it of 
necessity appears to be of the same size. The line d c, in Jig. 90, is four times lon^'er 
than a b ; but it is also four times more distant from F, supposed to be an observer's 
eye; consequently it has the same angular measurement, and seems to be of the same 
size. What is true of four times must also be true of 400 times, if both distance and 
size are increased in the same proportion. 



I 



128 



BOUVIER S FAMILIAR ASTRONOMY. 



Q. Is the ENTIRE DISC of the Sun hidden whenever the Moon intervenes be 
tween it and the Earth ? 

A. The entire face of the Sun is hidden only when the centi-es 
of the Moon and Sun fall in a straight line with the observer's eye. 



Fig. 91. 




If E {fig. 91) were an observer's eye, and M the Moon's position, no part of the Sun's 
face could be seen ; but if the observer's eye were at a, (out of the line connecting the 
centres of the Sun and Moon,) then all that part of the Sun's disc below b would be 
visible, and all that part above 6 would be concealed. 

Q. How many kinds of solar eclipses are there ? 

A. Three : partial, total, and annular. 

An eclipse is said to be central when the centres of the Sun, Moon, and Earth are in 
one line. This is always the case in total and annular eclipses. 

Q. What is a partial eclipse of the Sun ? 

A, An eclipse is said to be partial when only a p)ortion of the 
eclipsed orb is hidden from view. 

In fig. 91 the observer at a would see a partial eclipse. 



Fig. 92. 




Fig. 92 represents a partial eclipse, in which only nine 
di^rits of the Sun are obscured. 



Q. When an eclipse is only partial, how can its extent be recorded ? 

A. The disc of the Sun is supposed to be divided into ttoelve 
equal parts, called digits ; and the number of the parts eclipsed 
is recorded as so many digits. 

Q. What is a total eclipse ? 

A, An eclipse is total when the whole hody is obscured. 

Q. What appearance is presented to the eye during a total solar eclipse ? 

A. A round. Mack centre, sometimes surrounded by a halo of 
faint light, appears in the place of the Sun. 



ECLIPSES. 

Fiff. 93. 



129 







■Oii'r I I 



1 1 iffi 



The appearance of a total eclipse of the Sun is shown in Jig. 93. 
Q. What is the halo of faint light called ? 

A. The halo of faint light which surrounds the dark body of 
the Moon is called the corona. 

Q. What CAUSES the corona ? 

A, Astronomers are undecided as to the cause of the corona. 

Some suppose it to be portions of the solar atmosphere, or of the solar beams, bent by 
the influence of the Earth's atmosphere ; others suppose it to be the atmosphere of the 
Moon, which, though extremely rare, becomes visible during an eclipse of the Sun. 
But there seems to be objections to both these conjectures. 
Q. How LONG can a total eclipse continue ? 

A. A total solar eclipse, at any spot on the terrestrial globe, 
cannot last longer than from three to four minutes. 

Q. What is the phenomenon attendant on a total eclipse ? 

A. During a total solar eclipse, a deep and gloomy twilight 
takes the place of daylight, and the brighter stars appear. 

" 'Tis but the daylight sick; 
It looks a little paler; 'tis a day 

Such, as a day is, when the Sun is hid." — Merchant of Ybnice. 
Q. Is the Sun eclipsed 'over a large portion of the Earth's surface at once ? 

A. No ; the Moon's shadow is always small where it touches 
the Earth's surface. 

The greatest diameter of the Moon's shadow, where it touches the Earth, is only about 
one hundred and seventy-five miles; consequently, those inhabitants of the Earth who 

9 



130 bouvier's familiar astroxomy. 

are beyond this circle of one hundred and seventy-five miles of shadow see the Sun 
shining as usual. 

The phenomenon of a solar eclipse may be compared to that produced by a small 
cloud passing between us and the Sun. The cloud casts a shadow on the Earth as it 
moves along, and to those observers on whom the shadow falls, the Sun is for a time 
obscured, while those beyond the shadow enjoy the sunshine. Just so does the 
Moon's shadow fall on the Earth, and for a time obscures the Sun, while beyond its 
limits the inhabitants see him as usual. 

Q. Wliy is the Moon's shadow so small when it conies in contact with the 
Earth ? 

A. Because the body of the Moon being so much smaller than 

that of the Sun, it just hides the Sun to an observer on the 

Earth ; therefore, the Moon's shadow must end in a j^oint near the 

Earth's surface. 

Q. When a large luminous body shines on a small opaque one, what kind of a 
shadow is cast ? 

A. The shadow must be a diminishing one, in the form of a cone. 

During a solar eclipse a small circle of darkness, formed by the point of the lunar 
shadow, creeps along over the surface of the Earth. 
Q. What is an axxular eclipse ? 

A. An annular eclipse of the Sun is a total obscuration of the 
centre of that luminary, leaving only a bright ring of his disc 
visible. 

Q. When does an annular eclipse of the Sun occur ? 

A. An annular eclipse of the Sun occurs when the Earth is in 

perihelion, or in that part of her orbit nearest to the Sun, and 

when the Moon is in apogee, or in that part of her orbit farthest 

from the Earth. 

Q. Cannot a total eclipse occur when the Earth is in perihelion and the 
Moon in apogee ? 

A. No ; when the Earth is in her perihelion, the Sun's disc ap- 
pears of its greatest magnitude ; and when the Moon is in apogee, 
her disc appears of its least magnitude ; in that case the Sun's 
disc appears larger than the Moon's ; therefore, a total eclipse 
cannot occur. 

The reason why the Sun appears of its greatest magnitude when the Earth is in peri- 
helion, is, that the nearer a body is to us, the larger it appears ,• thus, the Sun appears 
of its greatest size when the Earth is in perihelion, or in that part of her orbit nearest 
to the Sun. And as the Moon is at her greatest distance from the Earth at her apogee, 
she consequently appears smaller than when at her perigee, or that point of her orbit 
nearest to the Earth. 

Q. If the eclipse cannot be total, what kind of eclipse will take place ? 

A. An annular 



Q. What is the appearan'ce produced by an annular eclipse ? 

A. The body of the Moon is seen to cover the centre of the 
Sun, leaving a luminous annulus or ring round its edge. 

Q. Does the point of the Moon's shadow fall on the Earth at the time of an 
annular eclipse ? 

A. No ; when an annular eclipse occurs, the point of the 
Moon's shadow /a/f?s a little short of the Earth's surface. 



ECLIPSES. 



131 



Fix. 94. 




Fig. 94 represents the Sun during an annular eclipse. 
Although eclipses of the Moon occur more frequently 
to the inhabitants of any one place, for instance, at 
Philadelphia, than eclipses of the Sun, yet as the Moon's 
shadow covers so small a part of the Earth's surface, 
the eclipses of the Sun can only be visible within a cir- 
cle comparatively small. But eclipses of the Moon are 
visible over a very large part of the Earth's surface, 
which is the reason why eclipses of the Moon are seen 
more frequently by a stationary observer than those of 
the Sun. 



Q. How MANY solar eclipses can there be in one year ? 

A. There are generally about four eclipses in a year — two of 
the Sun, and two of the Moon ; there may he as many as seven, 
but there must be two. When there are only two, they will both 
be solar, for eclipses of the Sun are more frequent than those of 
the Moon. 

Q. When can there not be any eclipse of the Sun ? 

A. When the Moon is so situated that at the time of new moon 
she is more than seventeen degrees from her node, no eclipse of the 
Sun can happen. 

Q. What is this distance of seventeen degrees called ? 

A. It is called the solar ecliptic limit ; that is, it is the far- 
thest possible distance from the node at which an eclipse of the 
Sun can take place. 



SECTION II. 

ITunar €dips£S. 

Q. Has not the Earth a shadow behind it as well as the Moon ? 

A. Yes ; the Earth's sphere is opaque as well as the Moon's 
sphere, and must also cast a shadow behind it. 
Q. What is the form of the Earth's shadow ? 

A. The Earth's shadow, like the Moon's, is of a conical form, 

Q. Why is the Earth's shadow conical ? 

A. Because the Sun is so much larger than the Earth ; and 
when a large luminous sphere illuminates a smaller one, the shadow 
must be conical. 

Q. Is the Earth's conical shadow longer than the Moon's ? 

A. The Earth's shadow is nearly /owr times longer than the 
Moon's shadow. 

Q. Why is the Earth's shadow nearly four times longer than the Moon's 
shadow ? 

A. Because the diameter of the Earth's sphere is nearly /oj^r 
times greater than the diameter of the Moon. 



132 



bouvier's familiar astronomy. 



The Moon's shadow extends about two hundred and forty-thousand miles from tho 
lunar sphere, and the Earth's shadow extends about eight hundred thousand miles from 
the terrestrial sphere. 

Q. Does the Earth's shadow extend, then, beyond the orbit of the Moon ? 

A. Yes. The Earth's shadow is nearly four times longer than 
the distance of the orbit of the Moon from us. 

Q. Does the Moon sometimes pass into the Earth's shadow ? 

A. As the Sun and Moon are sometimes situated in one straight 
line with regard to an observer on the Earth, therefore the Moon 
must occasionally be immersed in the Earth's shadow. 

Although the cone of the Earth's shadow is eight hundred thousand miles in length, 
there is no planet or satellite of the solar system (except our Moon) which is suflSciently 
near to be obscured by it. Consequently, our Earth cannot eclipse any of the other 
planets. 

Q. What HAPPENS when the Moon is in the Earth's shadow 

A. The Moon is no longer illuminated by the light of the Sun, 
and is then said to be eclipsed. 

Q. How is the Sun hidden in the solar eclipse ? 

A, By the dark body of the Moon coming m front of it. 

Q. How is the Moon hidden in a lunar eclipse ? 

A. In the lunar eclipse the Moon is hidden by coming into the 
dark shadow of the Earth. 

Q. At what PERIOD of the Moon's age does a lunar eclipse occur ? 

A. A lunar eclipse can only occur at the period of full moon. 

Q. What is the breadth of the Earth's shadow when the Moon traverses it ? 

A. The shadow of the Earth at the distance of the orbit of the 
Moon, (two hundred and forty thousand miles,) is between five and 
six thousand miles broad. 

Fig. 95. 




A cone that is eight thousand miles across at its base, and eight hundred thousand miles 
from base to apex, would measure between five and six thousand miles in breadth at the 
distance of two hundred and forty thousand miles from the base. Let S (Jirj. 95) be the 
Sun, and E the Earth ; then the length of the Earth's shadow, measured from E to a, is 
eight hundred thousand miles ; and the breadth of the shadow from b to c, where the 
Moon traverses it, is about five thousand six hundred miles. 

Q. How LONG does it take the Moon to travel through the Earth's shadow ? 

A. The Moon cannot be totally eclipsed more than an hour 
and three-quarters. 

A lunar eclipse may continue for five hours and a half, from the Moon's first entrance 
into the Earth's penumbra until her emersion from it. 



ECLIPSES. 133 

Q. Is the Earth's shadow bounded by an even, sharp outline, when seen o\er 
the disc of the moon ? 

A. No ; the edge of the Earth's shadow is so gradually softened 
off, that no precise line can be marked as its limits. 

It is not possible to discern where the edge of the shadow really begins. More and 
more of the Sun's disc is concealed from the Moon in succession, so that the solar light 
is gradually and progressively, instead of suddenly, withdrawn from it. For instance, 
when the Moon is at 1 in her orbit 1 6 c 3, (Jig. 95,) it receives the solar rays that issue 
from 2, but not those that issue from 4; and when at 3, it receives those from 4, but not 
those from 2. Consequently, while passing from 1 to & and from c to 3, it is in regions 
of diminished light, but not in entire darkness ; during this time its disc appears 
dimmed, but not obscured. 

Q. What is the imperfect shadow called which borders the true shadow of the 
Earth ? 

A. The imperfect shadow which borders the true shadow of the 
Earth is called the penumbra, which means almost shadow. 

Q. What is the meaning of ubibra ? 

A. Umbra means shadow. 

Q. Are eclipses of the Moon as frequent as those of the Sun ? 

A. No; lunar eclipses are not as frequent as solar eclipses. 

Q. Are more solar eclipses than lunar eclipses seen from any one point of ob- 
servation on the Earth's surface ? 

A. No; lunar eclipses are more frequently seen than solar 
eclipses from any one point of observation. 

Q. As there are fewer lunar eclipses than solar eclipses, how can this be ac- 
counted for ? 

A. A solar eclipse is only visible over a small portion of the 
Earth's surface, but a lunar eclipse is commonly visible to a very 
large portion of it. Consequently, any stationary observer more 
frequently sees lunar than solar eclipses. 

Q. Does the Moon sometimes undergo partial eclipse like the Sun ? 

A. Yes ; it frequently happens that the body of the Moon 
passes the edge of the Earth's shadoiv in such a way that a part 
of her disc is in the shadow and the other part outside of it. 

Q. Is the Moon's surface quite hidden from human vision when under total 
eclipse ? 

A. It rarely becomes invisible, but may be faintly seen, of the 
color of tarnished copper. 

This reddish appearance is owing to the refraction of the Sun's rays in passing 
through our atmosphere. 

Q. Is it more easy to calculate beforehand the time of a solar, or lunar 
eclipse? 

A. It is more easy to calculate beforehand the time at which 
a lunar eclipse will occur. 

Q. Why is it more easy to calculate the time of a lunar eclipse ? 

A. Because the time of the beginning and ending of a lunar 
eclipse, and also the time of its continuance, are quite independ- 
ent of the observer s p)osition on the Earth's surface. 



16A. BOUVIER S FAMILIAR ASTRONOMY. 

Q. Is the observer's position important in the calculation of a solar eclipse ? 

A. In a solar eclipse the observer's position is an important 
element in the calculation. 

Q. Why are astronomers more generally interested in -watching solar than 

LUNAR ECLIPSES? 

A. Because the time of the beginning and ending of a solar 
eclipse can be more accurately noted than in the lunar ; and when 
the solar eclipse is total, it is accompanied with the sudden with- 
drawal of the solar influence from the Earth. 

Q. Can any kind of regularity be discovered in the recurrexce of eclipses ? 

A. Yes ; after an interval of eighteen years and two hundred 
and sixteen days, the same eclipses occur over again. 

More properly, the same eclipses occur again after a period of 6585 days. It is sup- 
posed that a knowledge of this fact may have enabled the ancient astronomers to foretell 
eclipses. This cycle, or period of 18 years, was termed saros by the Chaldeans. 

On the first day of March, 1504, a lunar eclipse occurred, a foreknowledge of which 
enabled Christopher Columbus to obtain supplies for his crew, who were in a sufiFerin;^ 
condition. He was at that time at the island of Jamaica; and after using all the means 
in his power to obtain relief from the natives without success, he threatened them with 
the displeasure of the Great Spirit, which he said would be manifested on that very 
evening by the privation of the Moon's light. At first they disregarded his threat, but 
■when the eclipse commenced, they loaded him with gifts, beseeching him to withdraw 
the calamity, and promising to befriend him ever after. 

SECTION III. 

transits. 
Q. What is the meaning of transit ? 

A. Transit means the change of place or passage of a planet 
over the disc of the Sun. 

This term is applied to the passage of the Sun over the meridian of a place, and also 
to the passage of a heavenly body across the field of view of a telescope. 

Q. Do ALL the planets transit the Sun's disc? 

A. To the inhabitants of the Earth the interior planets only 
appear to transit the Sun. 

Q. Why do the planets Mercury and Venus only appear to transit the Sun's 
disc ? 

A. Because the orbits of those planets are within the orbit of 
our Earth. 

Q. Under what conditions do transits occui' ? 

A. A transit can only occur when the planet Mercury or Ve- 
nus is very near its node. 

The node, it will be remembered, is that point of a planet's orbit which intersects the 
plane of the Earth's orbit. Were the orbits of Mercury and Venus in the same plane 
with the orbit of the Earth, they would transit the Sun's disc at every revolution. It 
is only when the planet is in, or very near, the plane of the ecliptic, that a transit can 
occur. 

Let H o be the points in which the orbit M o B « of a planet intersects the plane of the 
ecliptic E o C H ; the part o B » lies above the plane of the ecliptic, and the other half, 
71 M o, hehiw it. That point through which the planet passes in ascending above the 
plane of the ecliptic is called the ascending node, which in the figure is the point w. 



ECLIPSES. 



135 



the point n denotes the descending node, or point where the planet descends 
plane of the ecliptic, as it moves in the direction of the arrows. 

Fig. 96. 



below the 




Q. What is the appearance produced by a transit ? 

A. The body of the planet is seen to cross the Sun's disc like 
a dark spot. 

Q. What do transits serve to prove ? 

A. They prove that the planet is an opaque body, shining only 
by reflected light. 

Q. What TERMS do astronomers use to denote the ingress and egress of the 
opposite limbs of the planet ? 

A. Internal and external contact. 

Q. What is the meaning of internal contact ? 

A. The moment of the p>erfect projection of the planet on the 
disc of the Sun, before the appearance of a luminous line between 
the edge of the planet and the limb of the Sun. This is the first 
internal contact. The last internal contact is when the planet, 
having passed over the Sun's disc, has not yet protruded beyond 
his limb. 

Q. What is meant by external contact ? 

A. The moment the planet touches the Sun's limb, before any 
apparent indentation is perceptible. This is the first external 
contact. The last external contact is when the planet, having 
passed over the Sun's disc, its edge is still in contact with the 
Sun's limb. 

Place a silver dollar on the table, and a small gold one by its side. Draw the gold 
dollar towards the silver dollar, so that their edges may just touch. This is the first 
external contact. Now, continue to move the gold dollar over the silver one: the mo- 
ment the whole of the gold dollar is on the silver one, with their edges still together, is 
ihQ i\mQ 0^ t\\Q first internal coiHact. Continue to draw the gold dollar over the silver 
one, till their opposite edges meet: this is the point of the lust internal contact. Con- 
tinue the motion, and the edge of the gold dollar will protrude over the edge of the 
silver dollar, and finally will leave it entirely. The moment when the gold dollar is 
entirely off from the silver dollar, but their edges are yet together, is the moment of the 
last external contact. 

Q. Do transits occur at every revolution of the planet? 
A. They do not ; because the orbits of the planets Mercury 
and Venus are not in the same plane with the orbit of the 
Earth. 



136 bouvier's familiar astronomy. 



DIVISION I. — transits OF MERCURY. 
Q. In WHAT PART of tlie Earth's orbit are the nodes of Mercury's orbit ? 

A. In that part of the Earth's orbit through which she passes 
in the months of May and November ; that is, in the signs Taurus 
and Scorpio. 

Q. How OFTEX do transits of Mercury occvir ? 

A, They usually occur at intervals of thirteen and seven years. 

Those transits at the ascending node occur in November; those at the descending 
node, in May. The intervals, considering each node separately, occur usually in the 
order of 13, 13, 13, 1, <fcc. ; and after a period of two hundred and seventeen years, the 
transits occur in regular order again. 

Q. How long since the first transit of Mercury was observed ? 

A. The first recorded transit of Mercury occurred on the 7th 
of November, 1631. 

Q. Who observed this transit ? 

A. An astronomer in Paris, by the name of Gassendi. 

Kepler, one of the greatest astronomers of that day, calculated the time this transit 
would occur, and notified astronomers to that effect. His calculations were proved cor- 
rect by the observations of Gassendi, who first thought the planet was a spot on the 
Sun ; but soon found by its motion that it was the body of the planet Mercury projected 
on the Sun's disc. 

Q. Are the transits of Mercury important to astronomers ? 

A, Yes ; but they are of less importance than the transits of 

Venus. 

Q. Why are they less important than the transits of Venus ? 

A. Because, as the planet Mercury is so near the Sun, it is 
more difficult to ascertain the Sun's parallax by means of the 
transit of that planet across his disc. 

Q. What is meant by horizontal parallax ? 

A. It is the angle subtended by the semi-diameter of the Uarth, 
if viewed from the Sun, Moon, or a planet. 

Q. Of what USE is the determining of the Sun's parallax ? 

A. It affords the means of ascertaining the distance of the 
Earth from the Sun. {See Note 35.) 

Parallax is explained in Part iii. chap. i. sec. i. div. vi. 



DIVISION II. TRANSITS OP VENUS. 

Q. Do transits of Venus occur as frequently as those of Mercury ? 

A. No ; they recur at intervals of eight and one hundred and 
thirteen years; but these intervals are by no means regular. 

Q. Are transits of Venus of any importance to astronomy ? 

A. Transits of Venus are of great importance, enabling astro- 
nomers to determine the distance of the Earth from tJie Sun 
with greater accuracy than any other known method. 



ECLIPSES. 137 

Q. How can the Sun's distance from the Earth be discovered by a transit of 

Venus ? 

A. The dark body of the planet appears to cross a different 
portion of the Sun's disc, as it is observed from one or the other 
of two widely-separated stations on the Earth's surface. 

Fig. 97. 




Let A B {jig. 97) represent the Earth, and v the planet Venus. If two spectators be 
placed on opposite extremities of the Earth's surface, one at A, and the other at B, 
when the observer at A sees the centre of the planet projected on the Sun's disc at d, 
at the same moment will the observer at B see it at c on the face of the Sun. Now, 
if these two observers mark the precise moment of ingress or egress of the planet, the 
angular measure of the distance c d can easily be ascertained by calculation. 

Q. Why is the transit of Venus viewed from two widely-sepakated stations ? 

A. Its path on the Sun's disc is noted hy each observer ; and 
from the amount of the displacement observed, owing to the 
effect of parallax, the relative distances of Venus and the Sun 
can be ascertained. 

The parallax of a celestial body is the angle under which the radius of the Earth 
would be seen, if viewed from the centre of that body. 

Q. Do transits of Venus happen every time the planet passes through that 
portion of its orbit nearest to the Earth ? 

A. They do not ; because the plane of the planet's orbit is not 
in the same plane as the orbit of the Earth. 

Owing to the inclination of the orbit of Venus to the plane of the ecliptic, she is 
sometimes above, and sometimes below, the Sun's apparent place. 

Q. Have transits of Venus been often observed by astronomers ? 

A. No ; the first recorded transit of Venus is that which oc- 
curred in the year 1639. 

This transit, supposed to be the first witnessed by any human being, was observed 
by a young man named Jeremiah Horrox, of Hoole, near Liverpool. He found the 
tables which Kepler constructed from the observations of Tycho Brahe were incorrect, 
inasmuch as they indicated a transit in the year 1631 : but as none appeared, he ap- 
plied himself to discover the error, and succeeded so far as to predict a transit in 1639, 
which actually took place. He was very anxious to witness this rare phenomenon, and 
for this purpose commenced his observations at a very early hour on the day of the ex- 
pected event. But as this was Sunday, he did not permit his desire to witness this rare 
phenomenon to interfere with his scrupulous observance of the day, believing it to be 
his duty to attend divine service twice. He was, however, in the interval, favored with 
the sight he had so long anticipated, and for which he had labored for eight years. 

Q. Have the transits of Venus been of interest to astroxomers only ? 

A. No; the transit of 1769 was considered of so much ira- 



138 bouvier's familiar astronomy. 

portance, that the British Government equipped an expedition 
on a large scale, and sent able persons to the Sandwich Islands 
to observe it. 

The expedition, under the command of Captain Cook, iras sent out to the island of 
Tahiti, by the British Government, to observe this transit. Other European powers 
made liberal preparations, and sent out astronomers to various parts of the world. 

Q. Were any peculiarities observed during that transit of Venus ? 

A. Yes ; a pyramid of light was observed on that side of the 
planet which was not yet in contact with the Sun, which increased 
until one-half the disc of Venus was projected on the solar disc. 

Fig. 98. 




The above figures represent the form of this light at its first appearance, and after 
t je centre of the planet had arrived at the edge of the Sun's disc. The drawing is from 
that made by Mr. David Rittenhouse, who observed this transit at his observatory at 
Norristown, near Philadelphia. 

A B {firj. 98) represents a portion of the orbit of Venus, and c {fig. 1 of the same 
diagram) gives the form of the first appearance of the pyramidal light. Fig. 2 shows 
the light as it appeared when the centre of Venus was in a line with the outer edge of 
the Sun's disc. It wiU be seen that it is then extended half round the orb of the 
planet. {See Note 36.) 

SECTION IV. 

^ctnltations. 
Q. "What is an occrLTATiox ? 

A. An occultation is the concealment of a planet or fixed star 
by another planet or the Moon. 

Q. To what is this coxcealmext of a planet or star owing ? 

A. The Moon passes between the Earth and the planets and 
stars, and hides them from our view ; for the Moon is much 
nearer to us than any other celestial body. 

Q. Does the Moon occult or conceal the more distaxt heavenly bodies as it 
passes between us and them ? 

A. Yes ; the Moon conceals planets or stars which lie directly 
in its path. 

Q. What is a star's obscuratiox by the Moon's opaque body calbd? 
A. The obscuration of a star by the Moon is called an occulta- 
tion of that star. 



ECLIPSES. J 39 

Q. Does the occultation of a star occur gradually, or instantaneously ? 

A. Planets are obscured by slow degrees; but fixed stars dis- 
appear instantaneously. 

Q. Why are planets concealed by slow degrees ? 

A. Planets have perceptible dimensions, and therefore the 
Moon's edge must require some time to pass over them ; but the 
fixed stars are without measurable discs, for being at such im- 
mense distances they appear as bright points, consequently the 
Moon conceals them entirely the very moment its edge appears 
in contact with them. 

"See; 
The Moon is up, it is the dawn of night ; 
Stands by her side one bold, bright, steady star." — Bailby. 

Pig. 99. 



Fic[. 99 represents the new Moon im- 
mediately before the occultation of a star. 
The star is about to disappear behind the 
unilluminated portion of the lunar disc. 




Q. What occurs when the occultation of a star happens before the period of 
full moon ? 

A. When the occultation occurs before the time of full moon, 
the star suddenly disappears behind the dark limb of the Moon, 
the unilluminated portion of the Moon being then towards the 

east. 

Q. What happens when the occultation occurs after the period of full moon ? 

A. After the period of full moon the occulted star reappears 
from behind the dark limb in the same sudden manner, the un- 
illuminated portion of the Moon then being turned towards the 
west. 



140 



CHAPTER XH. 

"Stranger cf Heaven, I cid tlaee Lail ! 

Siired from the pall of glory riTen, 
That fiashest in celestial gale — 

Broad pennon of the Xing of Heaven. 
TThate'er portends thy front of f re. 

And streaming locks so lovely pale 
Or peace to man, or judgm.ents dire, 

Stranger cf Heaven, I tid thee hail'' — Etthics Shepkehd. 

Q. Are the Moon and planets the only bodies seen to "wander about in the 
nocturnal sky ? 

A. There are yet other bodies which are occasionally seen to 
move among the stars. 

Q. What are those, xot planetary, ret wandering, bodies called ? 

A. They are called comets. The word comet is derived from 
a Greek word, signifying 'hai7\ 

Q. Is every part of a comet equaxly bright ? 

A. No; the central point is generally the brightest; this is 
called the nucleus or head, which is sometimes surrounded by 
a nebulous covering called the envelope or coma ; the luminous 
train by which most comets are accompanied is called the tail. 

The tail, however, is by no means an invariable appendage to comets. 
Q. How are comets distinguished from planets ? 

A. Comets present themselves with discs of cloudy light, and 
planets have well-defined discs. 

Xenocrates termed comets "clouds of light." 

Q. Are the motions of the comets directed around the Sun, as those of the 
planets are ? 

A. Yes ; the comets are supposed to move round the Sun in 
elliptical orbits, but of such great eccentricity that some are con- 
sidered by astronomers to be of infinite length. 

Q. What, then, is the nature of a comet's path through space ? 

A. The orbits of comets have mostly the form of very elongated 
ellipses. 

In the ages of superstition, comets were regarded as omens of some coming evil; and 
more than once have influenced the progress of sublunary affairs. 

"At His conamand, affrighting human kind. 
Comets drag on their blazing length behind ; 
'Not. as was thought, do they at random, rove, 
Eut in determ.ined times, through Icng sixipsss move " 



COMETS. 



141 




If a cone be cut in different directions, it may be 
made to form five different figures, known as the 
Conic Sections. Thus, in fig. 100, A is the apex of 
the cone, the dotted line A B the axis, and C D E P 
the base. As the axis is perpendicular to the base, 
the figure is a right cone. If a right cone, having a 
circular base, be cut through the axis from the apex 
to the base, the section will be a triangle; if cut 
through both sides by a plane parallel to the base, as 
g g, the section is a circle ; if the cone be cut slant- 
ing through both sides, the section will be an ellipsis, 
as i i ; if cut by a plane parallel to one of its sides, as 
pp, the section will be a parabola; and if cut by a 
plane parallel to the axis, it will be a hyperbola, 
as hh. 



Q. Where is tlie Sun situated in a comet's orbit ? 

A. The Sun is situated in one of the foei of the orbit. 

Q. Do comets that move in elliptical orbits ever return into sight again after 
they have once disappeared ? 

A. All comets that move in ellipses must reappear after a cer- 
tain length of time ; provided, that in the mean time no accidental 
disturbance influences their motions. But those comets which 
move in hyperbolic orbits can never visit our system more than 
once, {See Note 37.) 

Q. Are any comets known thus to reappear periodically ? 

A. Yes ; several comets are now known to return into sight at 
regular and ascertained periods. 

Q. Do comets on their return always present the same appearance ? 

A. No ; comets are liaUe to change their appearance^ so as not 
to be recognised again. 

Q. How can it be definitely known that comets ever return to our system ? 

A. A comet is only known to be the same hody^ on its return to 
our system, when the elements of its orbit are the same as those 
observed to belong to a comet which has appeared before. 

Q. What are the elements of an orbit ? 

A. Certain fixed quantities and facts are called elements, which 
are necessary in order to determine the form and extent of an 
orbit. 

Q. What elements are necessary to be identical in order to determine whether 
a comet has ever been observed before ? 

A. Five elements are required to be identical to prove the re- 
currence of a comet ; namely — 

1. The perihelion distance ; 

2. The longitude of the perihelion ; 

3. The longitude of the node; 

4. The inclination of the orhit ; and 

5. The time of perihelion passage. 



142 



BOUVIER S FAMILIAR ASTRONOMY. 



Q. What is the perihelion distance ? 

A, It is the distance of the perihelion point from the Sun, 
which is the nearest approach of the comet to that luminary. 
Q. What is meant by the longitude of the perihelion? 

A. The longitude of a planet or comet at the time of its least 
distance from the Sun, as viewed from the Sun's centre. 

Q. What is the longitude of the ascending node ? 

A. The longitude of the planet or comet as seen from the Sun 
when it is in its ascending node. 

Q. What is the inclination of the orbit ? 

A. The angle formed by the plane of the ecliptic with the plane 
of the orbit of the planet or comet. 

Q. What is to be understood by the time of perihelion passage ? 

A. The moment when a comet arrives at its least distance from 
the Sun. 

Astronomers usually adopt a fixed meridian of their own country in expressing the 
epoch of arrival at perihelion. 

Fig. 101. ^. Do any of these comets keep within the limits 

Perihelion. of our system ? 

A. Yes ; some of these comets revolve 
in orbits contained within the orbit of the 
planet Neptune. 

Q. By what name are these comets distinguished? 

A. The are known as '•''the comets of 
short period.'' 

Their names and periods are as follows : 

Encke's, completing its revolution in 3 years 108 days. 

De Vico's, 

Brorsen's, 

D'Arrest's, 

Biela's, 

Faye's, 

Besides these, several other comets are known to have 
- short periods. 

The orbits in which these comets of short period move 
lie in planes not very much inclined to the Earth's 
orbit. Brorsen's comet has an inclination of 31°, which 
is 3° less than the inclination of the orbit of the planet 
Pallas. They all move in their orbits in the same direc- 
tion with the planets. 

Fig. 101 represents the orbit of a comet whose period 
of revolution is 75 years. It will be seen that it extends 
beyond the orbit of Neptune. 




H 



5 


' 271 


5 


•- 2U 


6 


' 163 


6 


' 227 


7 


' 163 




Aphelion. 



COMETS. 143 

"Lo! from the dread immensity of space, 
Returning -with accelerated course, 
The rushing comet to the Sun descends ; 
And as he sinks below the shading Earth, 
With awf\il train projected o'er the heavens, 
The guilcy nations tremble." — Thomson. 

Q. Are all comets believed to move in elliptical orbits? 

A, It is well known that some comets do not move in ellipses. 
They ai-e known to have hyperbolic paths. 

Q. Do the comets which move in hyperbolic paths ever return into sight 
when they have once left the Sun ? 

A, They do not. They visit the Sun once^ and then sweep 
off into space, to return no more. 

"Where the train 

Of comets wander in eccentric ways. 

With infinite excursion through th' immense 

Of ether, traversing from sky to sky 

Ten thousand regions in their winding road. 

Whose length to trace imagination fails " 

Q. Have many comets been seen at different times in the heavens ? 

A. Yes ; several hundred comets have been seen at different 
times. 

Lalande enumerates no less than seven hundred. Arago believes that there cannot be 
less than seven milUom of comets passing within the influence of our planetary system. 
Kepler used to speak of the comets being as numerous as fishes in the sea. 

The following table, exhibiting the number of fully authenticated comets in each cen- 
tury, is from the recent work of Mr. J. R. Hind : 

Century, Observed in Europe 

•' and China. 

XL 36 

XII. 26 

XIII. 26 

XIV. 29 
XV. 27 

XVI. 31 

XVII. 25 

XVIII. U 

XIX. (first half) 80 



Q. Does every comet show itself with sufficient brilliancy to be perceptible to 
the naked eye ? 

A. A very small proportion of comets are ever visible to the 
naked eye. {See Note 38.) 

Immense numbers must approach near to, and recede from the Sun unknown to uj, 
owing to the heavens being obscured by clouds and fogs; besides, many traverse that 
part of the heavens above the horizon in the daytime, which would render them invisible 
to us. This has been proved; for according to Seneca, during a total eclipse of the Sun 
which happened B.C. 62, a large comet was seen in close approximation to that lumi- 
nary. Many comets have appeared which were visible by the naked eye iu the day- 
time; namely, those which appeared B. c. 43, a. D. 1402, 1472, 1532, 1744, and 1S43' 
A comet mentioned by Diodorus Siculus was so brilliant as to cast shadows during tb« 
uight as strongly marked as those formed by the Moon. 



Century. ^ 


bserved in Europe 
and China. 


I. 


22 


II. 


23 


III. 


44 


IV. 


27 


V. 


16 


VI. 


25 


VII. 


22 


VIII. 


16 


IX. 


42 


X. 


26 


Giving a total of 607. 





144 bouvier's familiar astronomy. 

Q. Do all comets move in the same dibection ? 

A. No. Comets move in all 'possible directions: some from 
west to east, like the planetary bodies ; others from east to west. 

Q. Are the paths of the comets, like the planets, confined to the zone of the 

ZODIAC ? 

A. No ; all except the comets of short period make their fitful 
appearances indifferently in all the regions of the visible heavens. 

Q. What FORM do comets most commonly assume ? 

A. When they are first seen, comets mostly appear as spherical 

or elliptical portions of light. 

Q. Is the aspect of each comet fixed and immutable like that of each indi- 
vidual planet ? 

A. No : the form and aspect of each separate comet undergoes 
continual change so long as it remains visible. 

Q. Is the visible surface of comets equally bbilliant in every part ? 

A. Most comets have comparatively bright centres, surrounded 
by an envelope having a cloudy or hairy appearance. 

Q. What is the brighteb centbe of a comet called ? 

A. The brighter centre is called the comet's nucleus. 

Q. When comets first make their appearance, are they bbight ? 

A. Comets generally come into sight as small species of light, 
which are barely visible with good telescopes. As they advance 
nearer the Sun, these specks become larger and brighter. 

Q. Of what kind of substaxce are comets formed ? 

A. Comets are composed of a transparent or translucent sub- 
stance^ much more ethereal than the finest wisp of cloud. 

Q. How is it KxowN that they are composed of a thin, transparent sub- 
stance ? 

A. Because generally stars of the faintest lustre remain per- 
fectly visible through the densest portions of comets, although a 
slight fog is sufficient to completely obscure the light of faint 
stars. 

Q. Are comets self-luminous, or do they shine by borrowed light ? 

A. Comets are supposed to shine by light which they have re- 
ceived from the Sun. 

Comets grow brighter, and their tails increase in length, as they approach the Sun, 
until they are lost in his rays; after they pass their perihelion, they assume their 
greatest splendor, and then gradually decrease in brilliancy — thus showing that their 
light depends upon their proximity to the Sun. Yet, under certain conditions, comets 
have been observed to increase in brilliancy, when, according to the theory of reflected 
light, their splendor should decrease. 

Q. Is the nucleus of a comet, as well as its body and tail, composed of a 
gaseous matter ? 

A. It is. When the nucleus of a comet is viewed through 
powerful telescopes, it becomes more and more misty, instead of 
being more clearly defined, as it would be, if composed of a shining, 
solid substance. 



COMETS. 145 

Q. What is the nucleus of a comet ? 

A. The brighter point in the centre of the head. 

In some instances, a very minute star-like point has been observed in the nuclei of 
comets, but it is a rare occurrence. These planetary nuclei, as they are called, consist 
of nothing more than nebulous matter in a high state of condensation, but which cannot 
be regarded as solid bodies. 

Q. Are the nuclei of comets ever very large ? 

A. Some have been of enormous dimensions. 

The following are the diameters (in miles) of the nuclei of some of the more remark- 
able comets since 1780 : 

Miles. 

Great comet of 1815, discovered by Gibers 5300 

1825 5100 

" 1843 , 5000 

First comet of 1780 4270 

" 1847, discovered by Hind 3500 

1819, (July) 3280 

Second " 1811, measured by Hcrschel 2610 

Great " 1807, " " 538 

" " 1811, " " 428 

Second" 1798, " Schroeder and Harding 125 

" 1805, known as Biela's 70 to 112 

The comet which was visible to the naked eye in June, 1845, had a bright nucleus, 
which must have been nearly 8000 miles in diameter, or about the size of our Earth. 

Q. Is the comet's substance as light, or free from weight, as it is thin and 
transparent ? 

A. It is the lightest ponderable substance known to exist in 
space. 

Q. What is the coma of a comet ? 

A. The nebulosity or atmosphere which surrounds a highly con- 
densed or planetary nucleus. The nucleus and coma^ taken to- 
gether, are called the head. 

Q. The nuclei of comets are sometimes of great dimensions, and as they are 
the CENTRE of the head, the comet's head must sometimes be of enormous size : 

is this THE CASE ? 

A. It is: the great comet of 1811 had a nucleus of four hun- 
dred and twenty-eight miles in diameter, and a head the diameter 
of which measured nearly jive tim.es the distance of the Moon 
from the Earth. This comet had the largest head of any on 
record. 

The diameters of the heads of some remarkable comets are given below: — 

Diiiraeter of Head. 

Great comet of 1811 1,125,000 miles. 

Halley's " 1836 357,000 " 

Encke's '' 1828 312,000 " 

First " 1780 269,000 " 

<' " 1846 •. 248,000 *' 

Lexell's " 1770 204,000 " 

Third " 1846 130.000 " 

Second " 1849 51,000 " 

First " 1847 25,500 « 

Fifth " 1847 18,000 " 

The size of the nucleus and. coma of a comet is not constant and invariable, but sub-- 
ject to sudden and great changes. As an instance of their variability. Mr. Macleir, in 
January, 1836, at the Cape of Good Hope, saw a woll-defined disc witliin the head of a 

10 



146 bouvier's familiar astronomy. 

comet, which, from the apparent diameter assigned, could not have been less than 
97,000 miles; but in the previous autumn, the very same comet had exhibited a bril- 
liant nucleus, varying at different times from 250 to 1000 miles in diameter. The nebu- 
losity of comets may extend much farther than we are able to distinguish from our 
point of observation on the Earth, and therefore our estimates of their true dimensiona 
are probably very often underrated. 

Q. Is the MASS of comets very great ? 

A. No : they have the smallest mass and largest volume of any 
members belonging to our system. 

Q. Do comets change their appearance as they approach the Sun? 

A. They do : in the larger comets the nucleus frequently grows 
smaller and brighter^ and beautiful trains of light are generally 
seen to issue from the head in a direction opposite to, or away 
from the Sun. 

Q. When do comets assume the greatest splendor of appearance ? 

A. Comets are more beautiful immediately after their passage 
near the Sun. They then shine with the brightest light, and have 
the longest tails. 

•• With E-weeping glories glides along in air. 
And shakes tlie sparkles from its blazing hair." — Pope's Homer. 

Q. Do all comets have tails ? 

A. The brighter comets generally have tails, but the telescopic 
comets are usually without that appendage, and appear only as 
roundish nebulosities. 

Q. Do comets generally have more than one tail ? 

A. No ; but instances are recorded in which more than one tail 
has been observed. 

The comet of 1744 is said to have had six tails ; but this is only on the authority of 
Cheseaux. The best observers of Europe make no mention of the phenomenon, which 
they certainly would have done had they witnessed a sight so astonishing. 

Q^ Are comets ever visible in the daytime ? 

A. Sometimes they are. The comet of 1843 was seen, the day 
after its perihelion passage, in full daylight, just before sunset, at 
Portland, Maine ; from the deck of the ship Owen Glendower, then 
off the Cape of Good Hope ; as well as at some places in South 
America and Southern Europe. 

Q. To what cause is the production of the comet's tail due ? 

A. The production of the tail is supposed to be due to the ap- 
proach of the comet towards the Sun. It is evidently caused by 
the direct influence of the Sun, though no satisfactory reason has 
been assigned for its formation. 

Q. What SINGULAR appearance has sometimes been seen in the tails of 
comets ? 

A. Apparent vibrations or coruscations, similar to the pulsa- 
tions peculiar to the Aurora Borealis, have occasionally been seen 
in the tails of comets. 



COMETS. 147 

Q. How do these vibrations appear ? 

A. They appear to commence at the head, and to traverse the 
whole length of the tail in a few seconds of time. 

Q. Is the CAUSE of these vibrations connected with the nature of the comet ? 

A. No ; these eflfects are attributed to the constitution of our 
atmosphere. 

The reason given by Dr. Olbers for this phenomenon is, that the different portions of 
the tail of a large comet must be situated at widely different distances from our Earth, 
so that light would require several minutes more to reach us from one extremity than 
from the other. But this reasoning has been proved futile, the pulsations being almost 
instantaneous. 

Q. Are the tails of comets formed suddenly ? ' 

A. They are frequently formed veri/ rapidly. 

The comet of 1843 had a tail which at one time extended 200,000,000 of miles from 
the head. If the comet had at that time been in the plane of the ecliptic, and close to 
the Sun, the tail would have extended far beyond the orbit of the planet Mars, and pro- 
bably have terminated in the region of the asteroids. Yet this wonderful appendage 
was formed in less than three weeks. 

Q. Can the Sun's attractive power be the cause of the production of the tail? 

A. It cannot; for gravitation would draw the tail towards the 
Sun, whereas that appendage usually shoots away from him. 

{See Note 39.) 

Q. Do comets pass from great extremes of heat and cold in their orbits ? 

A. They do ; for as comets move in more elongated ellipses 
than planets, they are subjected to greater variations of tempera- 
ture. When in perihelion, some comets are so near the Sun as 
to be exposed to a heat much more intense than that of red-hot 
iron ; and in aphelion they endure a temperature several hundred 
degrees colder than freezing water. 

"And feel by turns the bitter change 
Of fierce extremes ; extremes by change m.ore fierce ; 
From beds of raging fire, to starve in ice 
Their soft ethereal -warm.th." — Milton. 

Q. Do comets move very rapidly ? 

A, Some comets move with great rapidity, othefs more slowly. 
They move more quickly when near to the Sun, and more slowly 
as they recede from it. 

The comet of 1680 moved through its perihelion at the rate of 880,000 miles per hour. 

Q. Is it not surprising that bodies having so little mass as comets should move 
so rapidly? 

A. It would be surprising, if their motions were performed 
through a resisting medium like our air ; but the space through 
which comets travel is almost void. • 

Q Mer tion some of the most remarkable comets. 

A. The comets of 1680, 1811, 1843, and those known by the 
name of Halley's and Bielas comets, are the most remarkable on 
record. 



148 bouvier's familiar astronomy. 

Q. Ho-w large was the great comet of 1680 ? 

A. Its tail extended over more than 70° of the heavens, and 
measured, according to Newton, no less than one hundred and 
twenty millions of miles in length. 

It was the comet of 1680 which led Sir Isaac Newton to the study of cometary astro- 
nomy/. The following figure is a representation of that wonderful object: 

Fig. 102. 




COMET OF 1680. 

Q. What is the period of the comet of 1680? that is, how long does it require 
to perform one revolution round the Sun? 

A. According to the calculations of Professor Encke, it would 
require eight hundred and five years to perform one revolution 
round the Sun. 

If this calculation be correct, it may again be expected to be visible in our heavens 
nbout the year 2485 ! 

Q. HoAv FAR from the surface of the Stn was the comet of 1680 during its 
perihelion passage ? 

A. It was only about one hundred and fifty thousand miles 
from the Sun's surface. 

It was exposed to a heat, when nearest to the Sun, tir,e)ifi/-»eveu thousand times greater 
than that caftsed by his vertical niys in our torrid zone. It is probable that a degree 
of heat so intense would be sufficient to convert into vapor every terrestrial substam'o 
with which we are acquainted. 

Q. How LARGE would the Sun appear, if viewed at the distance of the comci 
when in perihelion ? 

A. The Sun's diameter, if seen from the comet at the distance 



COMETS. 



149 



of one hundred and fifty thousand miles, would appear about 
seventy degrees of angular measurement, or about one hundred 
and forty times larger than he appears to us. 

The disc of the Sun, if viewed from that distance, would nearly fill the whole extent 
of the heavens, from the horizon to the zenith. 

Q. How FAR from the Sun was the comet of 1680 supposed to be when at its 

APHELION? 

A. Not less than eiglity thousand millions of miles. 

The orbits of some comets have been computed whose aphelion distance can n«.t oe 
less than 400,000,000,000 miles; and there are many whose orbits doubtless expend 
much farther. 

"Hast thou ne'er seen the conaet's flaming flight? 
Th' illustrious stranger passing terror sheds 
On gazing nations ; from, his fiery train 
Of length en-irmous, takes his ample round 
Through depths of ether ; coasts unnumbered "worlds 
Of more than solar glory ; doubles wide 
Heaven's mighty cape ; and then revisits Earth 
From the long travel of a thousand years." — Touno. 

Q. Is the comet of 1811 one of short period ? 

A. No; the comet of 1811 is supposed to require more than 
three thousand- years for its revolution round the Sun. 

Q. Are any comets supposed to have a longer period than 8000 years ? 

A. Yes ; the comet which appeared in July, 1844, requires 
more than one hundred thousand years to perform its journey 
round the Sun. 

The perihelion distance of the comet of 1811 was 98,700,000 miles, while its aphelion 
distance was computed to be 40,121,000,000 miles, or fourteen times the distance of the 
planet Neptune from the Sun. 

Fig. 103. 




The above figure represents the comet of 1811, with its envelope, which has been 
rmcd a head-vail. 



lermc 



150 



BOUVIER S FAMILIAR ASTRONOMY. 



Q. What APPEARANCE did the comet of 1811 present? 

A. It presented a well-defined planetary disc ; that is, its nu- 
cleus was so bright^ and its edges so well-defined, that it had the 
appearance of the disc of a planet. 

Upon applying very high magnifying powers, Sir William Herschel found the nucleus 
to vanish, the light being much diffused, though not uniformly. 

Q. Was this bright nucleus accompanied by any other appearance ? 

A. It was involved or surrounded by a nebulosity^ which, to- 
gether with the nucleus, formed the head. 

Q. Describe the appearance of this nebulosity? 

A. This nebulosity was an envelope which surrounded the head 
like a vail. Its color was a blueish green, that of the nucleus be- 
ing of pale red. j,j„ ^^^ 




COMETS. 151 

The figure opposite represents the comet of 1843. It was observed at Chili, South 
America, to have a tail issuing from the side of the original one, at about 10° distance 
from the head, and extending to a much greater length than the first. — Comptea Rendua 
dea Sciencea, vol. xvii. p. 362. 

Q. For what was the comet of 1843 remarkable ? 

A. For the immense length of its tail, which was at one time 
computed to equal two hundred millions of miles ; besides which 
it had a very bright nucleus. 

Q. Describe the appearance of the nucleus ? 

A. It was small, but very bright, and of a golden hue, or red- 
dish, as seen by some observers. 

Q. Did the nucleus ever present a planetary appearance ? 
A. It did, when brightest, exhibit a planetary disc, which was 
estimated to be at least 5000 miles in diameter. 

Q. Did this comet approach near the Sun ? 

A. The nearest distance which this comet approached the sur- 
face of the Sun was ninety-six thousand miles, 

Q. With what rapidity did the comet of 1843 move when at its perihelion? 

A. It moved with the immense velocity of three hundred and 
sixty-six miles per second ! 

At that rate it would move 1,317,600 miles in an hour, which is five times the dis- 
tance of the Moon from the Earth. The whole of that segment of its orbit situated 
above the plane of the ecliptic in which the perihelion point was situated, was described 
by the comet in a little more than two hours. 

Q. Is the comet of 1843 supposed to be periodical ? 

A, It is; some of the best computations assign to it a period 
of three hundred and seventy-six years. 

Q. Why is Halley's comet so called ? 

A. It was thus designated in honor of Edmund Halley, a great 
English astronomer, who first accurately computed its elements 
in the year 1682. 

Q. Of what utility were these observations and calculations of Halley ? 

A. They served as data whereby to identify the comet with 
others which had appeared before. 

Q. How was this accomplished ? 

A. Halley compared its elements, that is, its perihelion dis- 
tance, its longitude of perihelion, longitude of its ascending node, 
inclination of its orbit, and time of its perihelion passage, with 
the elements of former comets. 

Q. What conclusion did Halley arrive at ? 

A. That it performs its journey round the Sun once in about 
seventy-five years, and believed it to be identical with the comets 
of 1456, 1531, and 1607. 

Q. Halley predicted that the comet of 1682 would return again in about 
seventy-five years; Avas his prediction verified? 

A. It was; the comet was seen ao;ain in 1T59. 



152 



BOUVIER S FAMILIAR ASTRONOMY. 



Halley's comet was the first whose return was successfully predicted, and whose orbit 
was accuratelj' determined. [See Xote 40.) 

As Halley could not hope to live to see his prediction fulfilled, he requested that 
astronomers might look for the comet at the designated time : and should it appear, ho 
was desirous that the world might remember that it was an Englishman who had fore- 
told its return. His name is associated with the comet, the discovery of whose period 
forms so memorable an epoch in the history of the science. 

Q. Has Halley's comet been seen sixce 1759? 

A. Yes ; in about 76 years after, that is, in the year 1835, it 
made its last appearance, and will not return to our view again 
until about the year 1911. 

Fig. 105. 




Q. Had Halley's comet a tail -when it was seen in 1835 ? 

A. When it was first seen in August of that year it had no in- 
dications of a tail; but towards the end of September a tail be- 
came gradually visible^ and by the middle of October of the same 
year, it attained its greatest length. 

Q. What APPEAKAXCE did Halley's comet present in the year 1835 ? 

A. It is described as having at one time the appearance of a 
powder-horn. Professor Struve compares the nucleus to 2^ fan- 
shaped flame, emanating from a bright point ; this flame after- 
wards changed its appearance, and resembled a red-hot coal of an 
oblong form. At another time it appeared like the stream of fire 
•which issues from a cannon's mouth after its discharge. 

At its nearest approach to the Sun this comet was 55,900,000 miles from his centre; 
and at its aphelion, or greatest distance from that luminary, it was 3.370,300,000 miles 
Irom his centre, which is a distance exceeding that of the planet Neptune. 



COMETS. 153 

Q. Who was the discoverer of Biela's comet ? 

A. It is not known who was the discoverer ; it was recorded 
as first seen in the early part of the eighteenth century. But it is 
thus called in honor of M. Biela, of Josephstadt, in Bohemia, who 
computed its elements, and found it to be a comet of short period. 

Q. What is the PERIOD of Biela's comet? 

A. It describes its journey round the Sun in a very eccentric 
ellipse, in about six years and seven months. 

Q. Is this comet a conspicuous object in the heavens ? 

A. No ; it is small, and hardly visible to the naked eye, even 
when brightest. 

Q. Why, then, is this comet remarkable ? 

A. It is worthy of note as being a periodic comet ; and also 
for the singular phenomenon which struck every astronomer with 
astonishment at its appearance in the year 1846. 

Q. What was this singular phenomenon ? 

A. It was seen to separate into two distinct parts or separate 
comets. 

Euphorus, a Greek author who lived more than 300 years before Christ, mentions a 
comet which, before vanishing, was seen to divide into two distinct bodies. Hevelius 
states that Cysatus perceived in the nucleus of the comet of 1618 an evident inclination 
to separate into fragments, which at one time bore a strong resemblance to an assem- 
blage of small stars. 

Q. Was the change observed in Biela's comet sudden ? 

A. Yes ; it was supposed to have separated within the space 
of two weeks. 

The change was discovered by Lieutenant Maury, at the National Observatory at 
AVashington, on the 12th of January, 1846 ; but was not observed in Europe until three 
days after, when it was seen by the Rev. Mr. Challis, of Cambridge, England, and M. 
Winchmann, of Koenigsberg. 

Q. How does Lieutenant Maury describe the appearance of this comet ? 

A. The comet which separated from the original one had a bright 
and starlike nucleus^ resembling a diamond spark. An appa- 
1 ent connection was maintained between the two, by means of a 
train of light from the larger to the smaller. 

Q. When this comet separated, did the two parts continue to journey together ? 

A. Yes ; the two parts moved along together for some time, at 
the distance of one hundred and fifty thousand miles asunder. 

At its appearance in 1852, the two parts of the comet were more than 1,250,000 miles 
asunder. This comet's greatest distance from the Sun is estimated at 590,100,000 miles, 
and its perihelion or least distance, at 81,600,000 miles. 

Q. Does the orbit of Biela's comet approach the orbit of the Earth ? 

A. By a remarkable coincidence, this comet, in the year 1832, 
came so near the Earth that if it had been retarded one month in 
its orbit, it would have come in collision with the Earth. But it 
did not approach nearer the Earth than several millions of miles. 

Sir John Herschel says — " The orbit of this comet approaches very near that of the 
Earth ; and had the latter, at the time of its passage in 1832, been a month in advam-e 
of its actual place, it would have passed through the comet — a singular rencontre, per- 
haps Qot unattended with danger." — HerscheVs Astronomy, p. 309. 



I'A 



BOUVIER S FAMILIA"^ ASTRONOMY. 



(^. What is there remabkable about the comet of 1770? 

A. This comet in its journey towards the Sun approached sc 
near the planet Jupiter, that it remained in its vicinity about four 
months ; yet it had no perceptible influence on Jupiter's satellites. 

Q. Did that comet approach very near the Earth ? 

A. Its approa<;h was so near the Earth, that had its mass been 
equal to the mass of the Earth, its attractive force would have 
been sufficient to have increased the orbit of the Earth, and con- 
sequently lengthened its time of revolution. 

Q. Had the comet's mass been equal to the mass of the Earth, how much 
would it have increased the length of our tear ? 

A. It would have increased the length of the year nearly three 
hours. 

Q. Did it lexgthen the period of our year ? 

A. No ; it did not produce any sensible effect on the length 
of the year; therefore its mass could not have exceeded the one 
five-thousandth part of the Earth's mass. Its nearest approach 
to the Earth did not have the slightest effect on our tides. 

Q. What NAME has been assigned to the comet of 1770 ? 
A. It is generally called LexelVs comet, from the great mathe- 
matician who labored in following and discovering its perturbations. 
Q. Did the orbit of Lexell's comet chaxge ? 

A. Yes ; previous to the year 1767 it moved in an elongated 
orbit, requiring nearly fifty years to accomplish one revolution 
round the Sun ; but owing to coming within the influence of Jupi- 
ter's attraction, its orbit was reduced to a small ellipse, in which 
the comet in the year 1770 revolved round the Sun m five and a 
half years. 

Q. In what kind of orbit does LexelFs comet move at present ? 

A. li h not hnoum; for owing to the superior attraction of the 
planet Jupiter, its orbit is entirely changed or deflected, and 
thereby is lost to our sight forever, unless it experiences some 
other change by approximation to one of the planets. {See Note 41.) 

"Lone traveller througii the fields of air, 
What may thy presence heje portend? 
Art come to greet the planets fair 
As friend greets friend? 

•• Whate'er thy purpose, thou dost teach 
Some lessons to the humble soul: 
Though far and dim thy pathway reach. 
Yet still thy goal 

• Tends to the fountain of that light 

From whence thy golden beams are won; 
So should we turn, from Earth's dark night. 
To God, our Sun." — Mrs. H.\t.8. 



SYSTExMS OF ASTRONOMY. 



155 



CHAPTER XIII. 

Q. What is meant by a system of astronomy ? 

A. The arrangement of the planetary system according to the 
theory of certain astronomers. 

Q. What is understood by a planetary system ? 

A. By a planetary system the ancients understood the disposi- 
tion and course of seven planets with regard to our Earth. 
Q. What is understood by the term solar system ? 

A. The Sun, with the primary and secondary planets and 
comets revolving around it. 

By a solar system, in a more extended sense, is understood any Jixed star with a num- 
ber of spheres revolving around it as planets. 



SECTION I. 

Q. Who was Ptolemy ? 

Ao He was an astronomer of Alexandria, who lived about 

A.D. 150. 

Q. Had he correct notions with regard to the arrangement of the bodies be- 
longing to our system ? 

A. He had not. 

Q. What BODY did he imagine to be the centre of the system ? 

A. He conceived the Earth to be the grand centre of the 
whole planetary system. 

Q. Was this system of astronomy first promulgated by Ptolemy ? 

A. No ; the system which is called Ptolemaic is wrapped in the 
obscurity of very Jiigh antiquity : its origin is unknown. 



Jj'ig. 106. — PTOLEMAIC SYSTEM. 



Ficj. 106 represents the solar system ac- 
cording to Ptolemy. The Earth is immov- 
ably fixed in the centre, around which are 
twelve circles. The first seven of these 
circles represent the orbits of the follow- 
ing bodies, thus: First, the Moon; next. 
Mercury; then, Venus; fourth, the Sun; 
fifth, Mars; sixth, Jupiter; and seventh, 
Saturn. The eighth circle, e, represents 
the sphere of the fixed stars ; the ninth, d, 
and tenth, c, called the first and second 
crystal heavens, were imagined by Ptole- 
my as necessary to account for certain phe- 
nomena which he could not otherwise ex- 
plain. The eleventh circle, b, he called the 
Primura Mobile, which, he supposed, car- 
ried the other ten circles enclosed in it, in 
its daily rotation from east to west; while 
the twelfth and last circle, a, Ptolemy indi- 
cated by the name of Empyreum, or the 
abode of spirits and the blessed. 




156 



BOUVIER S FAMILIAR ASTRONOMY 



Q. Was the system of Ptolemy bi;lieved in for a long time ? 

A. It was received bv many as the true system until the 
middle of the Urteenth centurv. 



SECTION IT. 

Z\i C^anptian .^nstcm. 

Q. What was the Egyptian system? 

A. The sy^ttm of the universe as believed in bv the Egyptian 
astronomers. 

The Ptolemaic system was denounced by the Egyptian astronomers as erroneous, 

because the sup>erior conjunctions of the inferior planets Mercury and Venus could not 
be accounted for by that theory. For a* (according to Pt-jlemy their urbits were be- 
tween the Earth and the Sun. there could be no reason assigned for their appearance 
beyond that luminary, or, what is the same thing, their superior conjunction. 

Q. Whfit was the cextre of the Egyptian system ? 

A: The Earth was the centre, and first the M^on and then the 
Suu revolved around it. 

Fk'.ljT. — litK ii<iTPTU.X SrSTKW. 



Fig. 107 represents the bodies of our 
STstem according to the theory of the 
Egyptian astronomers. The Earth is the 
ceatre. and the Muon and Sun revolve 
around it in circular orbits. Venus and 
Mercury, however, revolve round the 
Sun. and are carried with him around 
the Earth. Mars, Jupiter, and Saturn 
revolve in orbits far distant: and be- 
yond the orbit of Saturn is the region 
of the Used stiirs. 



SECTION III. 
Q. Who was the fouxdeb of the Tychonic system ? 

A. Tycho Brake, an astronomer who lived in the second half 
of the sixteenth century. 

Q. What was his theory of the solar system ? 

A. His reason dictated the theory of the Sun as the centre, 
and the planets, with our Earth, revolving round him. But he 
placed the Earth in a fixed spot, with the Moon revohdng round 
it, and the Sun as the centre of the orbits of the other planets. 

Tycho Brahe's reason for teaching against his better judgment was, that be believed 
it contrary to Scripture that the Earth should mjve. He therefore, like the ut) ei 




SYSTEMS OF ASTRONOMY. 



157 



theorists, placed in the centre of the universe, with the Moon revolving round it first, 
and then the Sun j round this latter body, the planets Mercury, Venus, Mars, Jupiter, 
aud Saturn revolve as their centre. Fig. 108.— tychonic system. 



^iCi$53%cC<N 



, Fig. 108 represents the solar system, 
according to Tycho Bruhe. 




Longomontanus, the friend and disciple of Tycho, supported an hypothesis different 
from either of those above mentioned, and approaching nearer to the truth. He main- 
tained, with Ptolemy, that the Earth was the centre of the universe; and with Tycho, 
that all the planets revolved round the Sun, which is placed in one of the foci of their 
elliptical orbits ; but he held that the diurnal motion of the heavenly bodies arose from 
the rotation of the Earth upon its axis, and thus got rid of one of the strongest objec- 
tions to the system of his iriend and master. 

SECTION IV. 

S^lje Copernttait ^gstcm. 

Q. When was the Copernican system first taught? 

A. It was known and taught hj Pythagoras, who flourished 
about B. c. 500. 

Q. Why is it called Copernican ? 

A. Because the system taught by Pythagoras was lost, and 
w^as not restored until the fifteenth century, when it was again 
taught by Nicholaus Oopernicus, a Polish philosopher. 

Fig. 109. — COPERNICAN SYSTEM. 



JJran u 5*^^^^ 



Fig. 109 represents the Irve, or Co- 
pernican, system, with the Sun in the 
centre, and the planets and comets 
revolving around him. 



Ju/iifriM sL^^^ij^^S^ 




158 bouvier's familiar astronomy. 

Q. Whrtt is the arrangement of the solar system as taught by Copernicus ? 
A. He taught that the Sun is the centre of the system, and 
that the planets^ including our Earth, revolve around him. 

Although this theory has been proved to be the true one, yet when first promulgated 
by Copernicus, it had many opponents. His substitution of the annual and diurnal 
niotiou of the Earth, they thought reasonable ; but his making the Earth and the whole 
terrestrial orbit shrink into a mere point in comparison wiih nearly all the other heavenly 
bodies and their respective orbits, was too sudden a demolition of long-nurtured opiniona 
to obtain extensive credit. But now the truth of this theory has been tested and firmly 
established by the observations of Galileo, Kepler, Newton, and others, although the 
theory as promulgated by Copernicus was afterwards much modified by the last-named 
astronomers. The system, however, still bears the name of the Copernican. 



PART III. 

SibtrnI ^stronomg. 



"And canst thou think, poor worm! these orbs of light. 
In Bize imxaense, in nifmber infinite, 
Were made for thee alone?" 

Q. What is Sidereal Astronomy ? 

A. Sidereal Astronomy treats of all those celestial bodies not 
included in our solar system. 

Q. What are those celestial bodies which are not members of our solar system 

CALLED ? 

A. They are generally known by the name of fixed stars. 

Q. Why are they called fixed stars ? 

A. Because they are fixed, or retain the same general positions 
with regard to each other. 

This is not strictly true; for all the fixed stars are supposed to have motion, though 
it is too minute to be perceptible, except with very delicate instruments, and by a series 
of observations continued through a long lapse of years. 

Q. How are the fixed stars distinguished from the bodies belonging to the 
solar system? 

A. They do not shine by borrowed light ; but, like our Sun, 
shine by their own native light. 

Q. How is it known that they shine by their own light ? 

A. Stellar, like solar light, is wholly unpolarized, while every 
reflected light, celestial or terrestrial, is possessed of such pro- 
perties as are acquired only through polarization. 

Q. Is there any other characteristic by which the fixed stars may be known ? 

A. The fixed stars may also be known by their scintillation or 
twinkling ; but the planets shine with a more steady light. 



THE FIXED STARS. lo9' 

The scintillation or twinkling of the stars is owing to the sudden changes in the refrac- 
tive powers of the different strata of our atmosphere, which, if they had perceptible 
discs like the planets, would not be sensible. 

Q. What is meant by the stellar universe? 

A. Bj the stellar universe^is meant all the celestial bodies not 
included in our solar system. 

Q. Are the bodies belonging to the stellar universe very remote ? 

A. Yes; they are situated so far from our Sun that their dis- 
tances are inconceivably great. 



CHAPTER I. 

f l]e im\ Stars. 

" The stars arc the landmarks of the universe ; and amid the endless and complicated 
fluctuations of our system, seem placed by their Creator as guides and records, not 
merely to elevate our minds by the contemplation of what is vast, but to teach us to 
direct our actions by reference to what is immutable, in his works." — Sir John Herschel. 

"Wliat involution! what extent! what swarms 
Of worlds ! that laugh at Earth ! immensely great ! 
Ijoamensely distant from, each others' spheres ; 
What, then, the wondrous space through which they roll 
At once it quite engulfs all human thought ; 
'Tis contemplation's absolute defeat." — Toung. 

Q. How many of the heavenly bodies which are visible to the naked eye may 
be considered as planetary bodies ? 

A, Venus, Mars, Jupiter, Saturn, and our Moon are the only 
planetary bodies which are usually visible to the unassisted eye. 

Q. What are the other stars visible to the naked eye called ? 

A. They are called fixed stars. 

Q. Why are they called fixed stars ? 

A. Because they are never seen to wander or move about 
among each other, like the planets and our Moon. 

Q. Are the fixed stars of any use to man ? 

A. They are a guide to the traveller by land, and direct the 
course of the navigator through the trackless ocean. They serve 
for "signs and for seasons, and for days and for years." 

By the observations of the relative situations of the stars, we are enabled to determine 
our days and years with the utmost accuracy. The stars are our most perfect chrono- 
meters. We have, by their assistance, measured the circumference of our globe, and 
determined the positions of all places on its surface. 

Q. Are the fixed stars without any motion with regard to each other? 
A. They are all subject to slight amounts of motion ; but it 
requires the nicest instruments to be able to detect it. 

Q. What are the apparent dimensions of the fixed stars ? 
A. The fixed stars are all ivithout apparent diameter. They 
are fixed points of light without measurable dimensions. 



' 160 bouvier's familiar astronomy. 

Q. Wliy are the fixed stars without measurable dimensions ? 

A. Their visible forms are mere points, too small to admit of 
angular measurement, even when seen through the most powerful 
telescopes. 

The eye is capable of perceiving very minute objects. Hueck states that spiders' 
threads, that measure less than a second in diameter, may be distinguished by the eye ; 
yet a star is entirely concealed ■when passing behind the finest spider's thread. Dr. Wol- 
laston contrived a means uf drawing out platinum wire so fine that one hundred and 
fifty might be bound together before one filament was produced as thick as the silk- 
worm's fibre ; and that thirty thousand of them could be laid side by side within the 
length of one inch ', yet Dr. Wollaston's wire can more than cover the point of even the 
brightest fixed star. 

Fixed stars are the smallest visible objects ever seen by the naked eye. They can be 
perceived, notwithstanding their v.ery minute dimensions, on account of the intense 
brilliancy of their light, and the absolute blackness of the background of space in which 
they appear. 

Q. Do ALL the fixed stars shine with equal brightness ? 

A. No ; the brightest fixed stars visible to the naked eye shine 
with tivo hundred times more light than the faintest. 

Q. How MANY stars does the naked eye perceive in the nocturnal sky ? 

A. About tivo or three thousand stars are visible to the naked 
eye ; but when we view the heavens with a telescope, their num- 
ber extends to 7iiany millions. 

The number of stars distinctly visible in both hemispheres in a twenty-feet reflector 
is nearly five and a half millions. That the actual number is much greater, there can 
be no doubt, when we consider that some portions of the milky way are so thickly 
studded with stars that it is impossible to count them. 

"Siu-vey this midnight scene; 
What are Earth's kingdoras to yon boundless orbs — 
Of human souls, one day, the destined range ! 

Q. How are the fixed stars classified as regards their brilliancy ? 

A, They are divided into orders or classes, thus : those which 
appear the brightest to the naked eye have been called stars of 
the first magnitude ; those which appear next in brilliancy, of the 
second magnitude ; and so on. 

Q. But are the brightest stars really larger and nearer than those which 
are less brilliant ? 

A. No ; the brightness of a star is no criterion whereby to 
judge of its distance. 

Q. Why, then, are the stars said to be of the first, second, third, &c. magni- 
tude ? 

A. The term magnitude is merely arbitrary, and is used by 
astronomers to indicate the apparent quantity of light emitted by 
those bodies. 

The term magnitude was not always used, for we find Leonard Digges, who lived in 
the sixteenth century, speaks of " starres oiihQ first bjghte." 

Q. How many of these stars are of first-class brightness ? 

A. About twenty of the fixed stars are of the first-class, or stars 
of the first magnitude. 



THE FIXED STARS. 



161 



I 



Q. What are the smallest stars discernible by the naked eye ? 

A. No stars smaller than the sixth class can be discerned by 
ordinary vision without a telescope. 

Q. Are the stars which cannot be seen by the naked eye classified ? 

A, Many of them are : all stars smaller than the sixth magni- 
tude are called telescopic stars, 

Q. What ARE the fixed stars ? 

A, The fixed stars are immensely/ large bodies, shining, as the 
Sun does, by their own inherent light. 

The quantity of light emitted by many of the fixed stars proves that they must be 
situated at immense distances from our system. By the experiments of Dr. Wollaston, 
it has been found that the light of the Sun is equal to eight hundred thousand full 
moons, and about twenty millions of millions of times greater than that of Sirius, the 
brightest of the fixed stars. 

Q. How are the fixed stars designated from each other ? 

A. There are catalogues and maps made of all the principal 
stars, amounting to about 20,000 ; and the stars belonging to 
each group are designated by the letters of the Greek alphabet; 
after which the Roman alphabet is used ; and lastly, figures. 

The Greek Alphabet. 



a 


Alpha, 


a. 


P 


Beta, 


b. 


y 


Gamma, 


g. 


6 


Delta, 


d. 


£ 


Epsilon, 


e short. 


; 


Zeta, 


z. 


ri 


Eta, 


e long. 


d 


Tbeta, 


th. 


t 


Iota, 


i. 


K 


Kappa, 


k. 


\ 


Lambda, 


1. 


H- 


Mu, 


m. 



V 


Nu, 


n. 


1 


Xi,^ 


X. 





micron, 


short. 


Tt 


Pi, 


P- 


P 


Rho, 


r. 


S 


Sigma, 


s. 


T 


Tau, 


t. 


V 


Upsilon, 


u. 





Phi, 


ph. 


X 


Chi, 


oh. 


^ 


Psi, 


ps. 


(O 


Omega, 


long. 



Q. What are the fixed stars supposed to be ? 

A. The fixed stars are supposed to be suns like ours, and the 
centres oi planetary worlds, 

Q. If our Sun is one of the fixed stars, why does it appear so large and 
brilliant ? 

A. Because our Sun is comparatively near to us ; if it were re- 
moved to the distance of the nearest fixed star, it would appear 
only like a brilliant point. 



SECTION I. 

g^ppar^nt Potions aitb positions of l^c ^tars. 

"Te stars -which are the poetry of Heaven." — Btron. 
Q. What is the sky ? 

A, The visible portion of the expanse of space surrounding the 
Earth. 

Q. Why does the sky look as if it were a concave surface ? 

A. Because the eye sees to an equal distance in all directions. 

11 



162 bouvier's familiar astronomy. 

Q. On turning the eyes upwards, what is the boundary of vision ? 

A. The apparent surface of the sky. 

Q. Why does the sky appear blue in a cloudless day ? 

A. Its blue color, during the day, arises from the atmosphere 
reflecting the blue rajs of light. It is properly the air which is 
blue. 

Q. Why do the nocturnal heavens assume the appearance of a concave 
surface ? 

A. Because the stars seem all to lie at equal distances from 
the Earth. • 

Q. What causes the apparent motion of the heavenly bodies ? 

A, The apparent motion of the heavenly bodies is due to the 
real motion of the Earth. 

Q. Why do the Sun, Moon, and stars seem to move along continually, from 
east to west, in the concave surface of the heavens ? 

A. Because the Earth's surface, whereon the observer stands, 

is as constantly carried along in the opposite direction — that is, 

from west to east — in consequence of the Earth's rotation on 

her axis. 

Q. How FAR do the heavenly bodies seem to move their positions during the 
lapse of an hour ? 

A, The heavenly bodies apparently change their positions ^ifeen 
degrees of angular motion every hour. 

Q. How is it proved that the stars have an apparent angular motion of fifteen 
degi-ees per hour ? 

A. The Earth revolves on its axis once in twenty-four hours ; 
therefore in one hour it accomplishes the twenty-fourth part of 
its rotation. As the whole circle of rotation comprises three hun- 
dred and sixty degrees, the twenty- fourth part of three hundred 
and sixty degrees would be the amount it would move in one hour. 
The twenty-fourth of three hundred and sixty degrees is fifteen 
degrees, which is the Earth's hourly motion, and the ajrparent 
motion of the stars. 

Q. Do the stars seem to move in circles that rise vertically from the east 
and DESCEND vertically towards the west? 

A. It is only at the equinoctial regions of the Earth that the 
stars seem to rise vertically from the east, and descend vertically 
towards the west. 

Q. Why do not the stars seem to rise vertically from the horizon in any lati- 
tudes except the equinoctial regions ? 

A. Because the points round which the surface of the celestial 
sphere seems to revolve are not placed in our horizontal circle. 

Q. What are the points around which the celestial bodies appear to move ? 

A They are the poles of the heavens. 

Q. What ARE the poles of the heavens ? 

A. The celestial poles are the axis of the Earth extended to the 



I 



THE FIXED STARS. 163 

celestial sphere. They are situated ninety degrees from the equi- 
noctial. That conspicuous star within a degree and a half of the 
north pole is called the pole star. 

Q. Where would the poles of the heavens appear to an observer situated at tbe 

EQUATOR ? 

A. The poles of the heavens would appear in the horizon of an 
observer at the equator. 

Q. Where would the equator appear to an observer at the poles of the Earth ? 

A. The equator would be in the Jiorizon of an observer situated 
at the poles. 

Q. Why would the poles appear in the horizon to an observer at the equator, 
and the equator in the horizon to an observer at the poles ? 

A, Because the horizon extends 90° each way from an ob- 
server wherever he may be situated ; and as the poles are 90° 
from the equator, they would appear in the horizon of a person 
at the equator ; for the same reason the equator would appear in 
the horizon to a person at the poles. 

Q. Where would the pole appear to a person situated in latitude 45°, or half- 
way between the equator and the poles ? 

A, To a person half-w^ay between the equator and the poles 
the pole would appear half-ivay between the zenith and horizon, 

Q. What is the latitude of a place ? 

A. It is the distance from the equator north or south. 

Q. How may the latitude of a place be found ? 

A. By observing the altitude of the pole above the horizon, 

Q. Explain this, 

A. If the altitude or elevation of the north pole above the hori- 
zon be found to be 20°, the latitude of that place is 20° north; 
if the south pole be observed to be elevated 20°, the latitude is 
the same number of degrees south of the equator. 

Q. When the Sun is vertical at the tropic of Cancer, which is 23^° north of the 
equator, how does he appear to the inhabitants under the arctic circle ? 

A. About the middle of June, when the Sun is vertical at the 
tropic of Cancer, the inhabitants who live under the arctic circle 
see him nearly at the east point of their horizon at six in the 
morning ; at noon he will be near midway between the south point 
of the horizon and the zenith ; at six in the evening he will be 
near the west point ; and at midnight he will be seen on or near 
the north point of the horizon. 

Q. Do the apparent positions of the fixed stars undergo any changes ? 

A, Yes; their positions appear to change from several causes. 

Q. What causes the changes in the apparent positions of the fixed stars ? 
A. The apparent positions of the fixed stars are affected by ^iz 
separate causes of variation. 



164 



BOUVIER S FAMILIAR ASTRONOMY. 



Q. What are the six separate causes which produce a slight change in the 
apparent positions of the fixed stars ? 

A. The apparent places of the fixed stars are slightly changed 
by precession, nutation, refraction, aberration, proper motion, and 
parallax. 

DIVISION I. — PRECESSION. 
Q. "What is precession? 

A. A retrograde motion of the equinoctial points. 

Q. What are the equinoctial points ? 

A. The two points where the celestial equator is intersected by 
the ecliptic, are called equinoctial points. 

"WTien the Sun is in either of these points, the days and nights are equal to each other 
in every part of the globe. 

Q. Are the equinoctial points pixed with respect to the stars? 

A. No ; they possess a retrograde or westerly motion, contrary 
to the order of the signs. 

Q. What is this retrograde motion called ? 

A. It is called the precession of the equinoxes. 

Q. Why is it called the precession of the equinoxes ? 

A. Because the position of the equinox in any year goes before 
or precedes, in the order of the signs, the place which it occupied 
in the previous year. 

Q. What produces the precession of the equinoxes ? 

A. The motion of the Eartlis axis round the poles of the 
ecliptic. 

Q. What produces the revolution of the pole of the equator round that of the 
ecliptic ? 

A. It is produced by the combined action of the Sun and Moon 
on the redundant matter at the equator of the Earth, by which 
its figure is rendered spheroidal. 

Q. At what RATE is the retrograde motion of the equinoxes ? 

A. At the rate of about one degree in seventy years, or of 
fifty seconds annually. 

Q. What EFFECT has the precession of the equinoxes upon the apparent posi- 
tions of the stars ? 

A. Precession increases the longitudes of the stars. 

Q. Why does precession increase the longitudes of the stars ? 

A. Because longitude is reckoned from the point called the 
vernal equinox, and a retrogradation of this point on the ecliptic 
must increase the angular distance of a star. 

Q. How LONG does it take for the pole of the equator to perform a revolution 
round the pole of the ecliptic ? 

A. This revolution, performed in a path scarcely difiering from 
a circle, requires no less than twenty -five thousand eight hundred 
and sixty-eight years, in which interval the equinoxes complete an 
entire circuit of the heavens. 



THE FIXED STARS. 
Fig. 110. 



165 




Suppose Q to he the pole of the ecliptic, and the small circle to be the path traced out 
by the pole of the equator in the course of its revolution around Q ; also let A B represent 
a part of the ecliptic, the arrows indicating the order of the signs, or the direction of 
the Sun's apparent motion. When the pole of the equator is at P 1 the equinoctial 
point will fall at El; but when the pole, in its westerly course, arrives at P 2, the 
equinox will have fallen back from E 1 to E 2. It will have moved contrary to the 
Sun's apparent motion, so that the point from which we reckon longitudes on the 
ecliptic, and right ascensions on the equator, must occupy a position among the stars, 
when the pole of the equator is at P 2, behind that which it possessed at the time the 
pole was at P 1 ,• and as the stars do not really participate in this movement, their longi- 
tudes are necessarily increased from year to year. 

Q. Was the pole star always situated mthm a degree and a half from the 
north pole ? 

A. No ; in the earlier periods of astronomical observation it 
was situated twelve degrees from the celestial pole. 

The pole star is at this time one degree, twenty-seven minutes from the true pole. 
Q. Will it ALWAYS retain its present position with respect to the pole ? 

A. No ; its position is always changing, though extremely 
slowly. It will continue to approach the pole until within half a 
degree of it, after which it will recede from the pole for thousands 
of years. 

In rather more than 12,000 years the bright star Wega, in the constellation Lyra, 
will be within five degrees of the pole, and will be the pole star of the northern hemi- 
sphere. {See Note 42.) 

DIVISION II. — NUTATION. 

Q. What is nutation ? 

A, The attraction of the Moon upon the spheroidal figure of 
the Earth gives rise to a slow motion of her axis to and fro, which, 
from its oscillatory character, has been termed nutation. 

Q. Dees nutation produce any effect on the equinoctial points ? 

A, It produces a slight advance and retiirn of the equinoctial 
points, occurring alternately about every nineteen years. 



166 bouvier's familiar astronomy. 

Q. What EFFECT has this alternate changing of the equinoctial points upon the 
apparent positions of the fixed stars ? 

A. It increases and diminishes the longitudes and right ascen- 
sions of the fixed stars alternately, in an interval of nearly nine- 
teen years. 

Q. What APPARENT MOTION does nutation produce ? 

A. It makes the pole of the heavens appear to revolve in a 
small ellipse once in about every nineteen years. 

Q. Why does the celestial pole appear to perform a revolution once in nine- 
teen YEARS ? 

A. Because the Moons attraction goes through all its varia- 
tions once in nineteen years, which period is called a c7/cle of the 
Moon. 

Q. But owing to the precession of the equinoxes the pole of the equator moves 
round the pole of the ecliptic : has the equatorial pole any other motion ? 

A. Yes ; it undergoes the oscillatory motion arising from the 
nutation of the Earth's axis ; whence it appears its true course 
will be in a wavelike curve. 

Fig. 111. 



Fig. Ill represents, though highly exaggerates, 
this wavelike curve, the centre of which marked P, 
is the pole of the ecliptic. {See Note 43.) 




DIVISION III. — refraction. 

Q. What is the atmosphere ? 

A. The fluid which we breathe, and which surrounds the globe. 

Q. What is the height or thickness of the atmosphere ? 

A. Its limit is not accurately known, but it cannot be less than 
forty miles high. 

Q. Is the density of the atmosphere the same throughout its "vrhole extent ? 

A. No; its density diminishes as its height increases; that is, 
it is more dense near the surface of the Earth than at a consider- 
able elevation above it. 

Q. Why is the knowledge of the constitution of the atmosphere important 
to the astronomer? 

A. Because, like all transparent media, it possesses the power 
of refracting or bending the rays of light out of a straight course. 



THE FIXED STARS. 



167 



Q. What EFFECT is produced by the rays of light being refracted or bent? 

A. Objects seen obliquely through the atmosphere do not ap- 
pear in their true places, 

Q. How can this be EXPLAINED? 

A. It is an established law in optics, that when a ray of light 
passes from a rare medium into one of greater density, it is de- 
flected or bent from its original course. 

Q. When a ray of light passes from a rare medium into one of greater density, 
m what DIRECTION is it bent ? 

A. It is bent more and more towards the perpendicular as the 
medium through which it passes increases in density. 

Q. What is the effect produced on a ray of light passing through the Earth's 
atmosphere ? 

A. As it approaches the surface of the Earth it continually en- 
counters a denser medium than that which it has already passed 
through ; consequently it is bent more towards the perpendicular, 

Q. When a ray of light from a heavenly body reaches the eye of a spectator, 
does the body appear in its true place ? 

A. No ; it appears higher up from the horizon than it really 
is, owing to the rays of light being bent more towards the per- 
pendicular. 

Fi-. 112. 




Let a spectator be placed at A, any point of the Earth's surface, and let L M N repre- 
sent the successive strata of the atmosphere, increasing in density as they approach the 
Earth. Let S be a star or other heavenly body, far without the limits of our atmo- 
sphere; if the air were removed, the spectator would see it in the direction of the 
straight line A S. But when the ray of light S A reaches the atmosphere at d, it will, 
by a law of optics, be bent downwards, and take a more inclined direction, as d c. and 
reach the Earth at the point a. But this ray will not strike the eye of the spectator. 



168 bouvier's familiar astronomy. 

The ray which will enable him to see the star is not S d A, but another ray, which, had 
there been no atmosphere, would have struck the earth at K, a point behind the specta- 
tor, but which, being refracted by the atmosphere into the curve S D C A, actually 
strikes on A. An object is always seen in the direction which the visual ray has at the 
instant of arriving at the eye. Hence the star S will be seen at T in the line A T, a 
tangent to the curve S D C A. Thus it will be seen that the refractive power of the 
atmosphere causes all heavenly bodies to appear more elevated than they would were 
there no atmosphere. Therefore the Sun when at P, below the horizon II A of the 
spectator at A, appears in the line A F, being bent in the curve P g' r A, to which A F 
is a tangent. 

Q. What is the phenomenon of seeing a heavenly body in a direction different 
from the true one called ? 

A. It is called refraction, 

Q. Do celestial objects ever appear to us in their true places ? 
A. Yes ; those bodies immediately/ overhead always appear in 
their true place, for in the zenith there is no refraction. 

Q. Why is there no refraction of the rays of light coming from an object in 

the ZENITH ? 

A. Because a ray of light, coming from the zenith, falls per- 
pendicularly on the surface of the Earth ; and as refraction only 
tends to hend it towards the perpendicular, it follows that an ob- 
ject situated in the zenith is seen'in its true place, 

Q. Where is the amoimt of refraction the greatest ? 

A. The amount of refraction is the greatest at the horizon. 

Q. What is the amount of refraction at the horizon ? 

A. The amount of refraction at the horizon is rather more than 
thirty-five minutes of angular measurement. 

Q. Explain this. 

A. Any object — a fixed star, for instance — when really heloio 
the horizon, would appear to be thirty-five minutes above it, and 
consequently still in sight. 

Q. Do the Sun and Moon appear above the horizon after they really 
have set ? 

A. Yes ; both the Sun and Moon appear to be wholly above 
the horizon when they are really below it. 

Q. What is the effect, then, of refraction ? 

A. It hastens the apparent rising of the Sun and other hea- 
venly bodies, and delays their apparent setting, beyond the true 
time ; thus increasing the duration of daylight. 

Q. Why do the Sun and Moon look oval, instead of round, when near the 
horizon ? 

A. This is owing to refraction: they do not appear round, 
because the lower limb or edge is raised by atmospheric re- 
fraction. 

W^hatever increases the density of the atmosphere augments its refractive power. 
■ySee Note 44.) 

" O'er yonder eastern hill the twilight pale 
Walks forth from darkness." — Akbnside. 



THE FIXED STARS. 169 

Q. What is twilight, or the crepusculum ? 

A. The faint light which may be perceived a short time before 
Bunrise and after sunset. 

Q. Do we see only by the direct light of a luminous object? 

A . No : if an^ portion of the direct rays from a luminous body 
are intercepted in their course, and thrown back upon us, it serves 
as a means of illumination. 

Q. How can a luminous body enlighten us, if the direct rays from it cannot 
BEACH us ? 

A, The atmosphere sends us a portion of the light from the 
luminous body, not by direct transmission, but by the refraction 
of its rays, and the reflection of them upon the vapors and solid 
particles which float in the air. 

Q. Why is a single sunbeam let through the crevice of a close shutter suffi- 
cient to prevent entire darkness in an apartment ? 

A. JBecause the atmosphere has the property of reflecting a 

portion of its light. 

Q. What are those lines of light sometimes seen in the atmosphere, vulgarly 
called " the Sun drawing water?" 

A. Those lines of light are the direct rays of the Sun shining 
through some broken clouds ; but the light from them is reflected 
by the atmosphere. 

Q. Do the Sun's rays fall upon the atmosphere after he has sunk below the 
horizon ? 

A. They do ; and the light is refracted and reflected by the 
atmosphere, and thrown upon the Earth, producing twilight. 

Q. How LONG does twilight usually continue ? 

A. Its duration depends very much upon the state of the atmo- 
sphere ; but, under ordinary circumstances, twilight ends in the 
evening and commences in the morning when the Sun is eighteen 
degrees below the horizon. 

" The tender twilight with a crimson cheei: 
Leans on the breast of Eve." 

Q. What happens in high latitudes, when, for a portion of the year, the Sun 
never sinks more than 18° below the horizon ? 

A, To those places twilight never ends from sunset to sunrise ; 
that is, there is no night, 

DIVISION IV. — ABERRATION. 
Q. What is meant by aberration ? 

A. The difference in the apparent motion of the fixed stars, 
caused by the velocity of the Earth combined with the velocity of 
light. 

Q. Is light propagated instantaneously from a luminous body ? 

A. No ; light requires time to travel from the luminous body 
which emanates it. 



170 bouvier's familiar astronomy. 

Q. Does light require time to travel from the heavenly bodies to the 
Earth ? 

A. Yes ; light travels at the rate of one hundred and ninety- 
two thousand miles in a second of time ; therefore we see 
the heavenly bodies by rays which left them some time 
before. 

Q. How was it discovered that light requires time to travel through space ? 
for if a candle be lighted in an apartment, no perceptible time is required to illu- 
minate every part of it. 

A. It was discovered by a comparison of observations made on 
the eclipses of Jupiter s satellites. 

The distance across an apartment is too small to admit of the measurement of 
the time required by the light to travel from one extremity of it to the other. The 
time which light requires to travel even a mile is too small to be appreciable by 
any means of measurement which we possess ; but it is measurable at the distance of 
Jupiter. 

Q. Who DISCOVERED the progressive motion of light ? 

A, Boemer, a Danish astronomer, in the year 1667. 

Q. How was Roemer led to this conclusion ? 

A. He found that when Jupiter was opposite to the Sun, or in 
that part of his orbit nearest to the Earth, the eclipses of his 
first satellite occurred sooner by sixteen minutes and twenty-six 
seconds, than when he was in opposition, or in that part of his 
oMt farthest from the Earth. 

Q. What did he infer from this discovery ? 

A. He concluded that the light reflected by Jupiter required 
sixteen minutes and twenty-six seconds to travel over the diameter 
of the Earth's orbit ; therefore it would require eight minutes and 
thirteen seconds to travel half that distance, or from the Sun to 
the JSarth. 

This theory at first met with violent opposition, but it was afterwards confirmed. 

Q. What OTHER DISCOVERY Confirmed the theory of the successive transmission 
of light ? 

A. The discovery of the aberration of light, by Dr. Bradley, 
in 1727. 

Q. What is the general effect of aberration upon the appearance of the 
stars ? 

A. It causes each star to describe (in appearance only) a very 
small ellipse in the course of the year ; the central point of the 
ellipse being the place the star would seem to occupy if our Earth 
were at rest. 

Q. Why does aberration cause the apparent places of the stars to differ from 
their true ones ? 

A. Owing to the motion of the Earth in her orbit, the rays 
emanating from a star appear to reach us from a different direc- 
tion to what they would were the Earth at rest. 



THE FIXED STAKS. 



171 



Fig. 113. 




Let B B be a part of the Earth's orbit, and 
D the place of a star. Now, in order to see 
the star, the telescope must be inclined in the 
direction B E ; for if it were pointed to the true 
place of the star D, the rays of light, instead of 
falling through the tube to the spectator's eye, 
would strike against its sides. But as the 
Earth moves in the direction denoted by the 
arrow, the rays of light from D will appear to 
come from the point E. The angle D B E, con- 
tained between the axis of the telescope and a 
line drawn to the true place of the star, is its 
aberration. 



Q. Suppose, then, we wish to observe a star; do we point the tube of the 
telescope to the star's true place ? 

A. We do not; but incline it a little, so as to admit the ray 
of light from the star to the observer's eye, without striking 
against the side of the tube. 

M. Clairaut, in the Memoirs of the Academy of Sciences for the year 1746, illustrates 
the effects of aberration in a familiar manner, thus : Suppose drops of rain, blown by 
the wind, to fall rapidly one after the other, and a person walk out, holding up a very 
narrow tube : it is evident that the tube must have a certain inclination, in order that 
a drop which enters at the top may fall freely through the axis of the tube without 
touching its sides ; which inclination must be more or less, according to the velocity of 
the drops in respect to that of the tube. The angle made by the direction of the tube, 
and falling drops, is the aberration arising from the combination of those two motions. 

Q. When is aberration at its maximum ? 

A. When a ray of light is perpendicular to the direction of the 
Earth's motion; but when it is parallel to the Earth's motion, 
aberration vanishes altogether. 

Q. The fixed stars are at rest, or nearly so, with respect to the Earth, but the 
planets and comets are constantly changing their positions : are they, therefore, 
SIMILARLY AFFECTED by aberration ? 

A. There is a distinction between the aberration of the fixed 
stars and what is termed the aberration oi planets and comets ; for 
the moment a ray from a moving body reaches the Earth, the 
true position of that body is changed by its actual motion in its 
orbit. 

Q, In what direction do objects always appear ? 

A. In the direction of the rays which render them visible. 



Q. Are celestial objects always situated in the direction of the rays which 
proceed from them ? 

A. They are not; but they would be if light were propagated 
instantaneously. 

Q. How is the true place of a moving body to be found ? 

A. The distances of all the planets are known to us ; and as we 



172 boijVier's familiar astronomy. 

also know the rate at which light would require to travel from 
them to us, it is only necessary to find how far the Earth has 
moved in the time, to determine the effect of the aberration, and 
consequently to discover the true place of the body. 

Q. Do we evei' see the Sun in his true place in the ecliptic ? 

A. No ; for light requires 8' 13" to travel from the Sun to 
the Earth, and in this Interval the Earth has moved over an arc 
of 20''*5, so that the Sun will appear 20''*5 behind his true place 
in the ecliptic. (See Note 45.) 

DIVISION V. — PROPER MOTION. 

Q. What is meant by proper motion ? 

A. An apparent motion of the fixed stars in certain deter- 
minate directions in the heavens. 

Q. What is the cause of the proper motion of the stars ? 

A. Two causes have been assigned : one is, that each star may 
have a motion to the observed amount; the other, that the solar 
system may have a motion in a contrary direction to the apparent 
motion of the stars. 

Q. Are these motions of our Sun or the stars certainly known ? 

A. The proper motions of many of the stars are known,, and it 
is believed that our solar system has a motion in space round some 
unknown centre. [See Note 46.) 

It would require centuries of observations to solve this highly interesting problem, 
and to find the elements of the grand orbit of the solar system. 

Q. Do any of the stars appear to have a more rapid motion than others ? 

A. Yes ; some few have attracted the attention of astronomers 
from their rapid motion, as compared with the rest. A star of 
the sixth magnitude, in the constellation Oygnus, numbered 61 in 
the catalogue, is moving at the rate of more than five seconds of 
arc annually. 

Q. What, then, is the probable velocity of the star 61 Cygni? 

A. According to the observation of its angular movement, its 
velocity cannot be less than about six thousand millions of miles 
in an hour! ! 

Q. Have not these inconceivable velocities changed the apparent places of 
the stars very considerably? 

A. Since the commencement of the Christian era, the apparent 
place of 61 Cygni has only undergone a change of about 2J°, 

Q. How can this apparent small change of place be accounted for ? 

A. The star is situated at such an immense distance from us, 
that although its motion is very great, it is imperceptible to us, 
unless we compare it with former observations. 

To a p<irson standing on an eminence on the sea-coast, a ship in the horizon or oflBng, 
moving at the rate of ten miles an hour, would appear stationary. Its motion would 



THE FIXED STARS. 178 

only be perceptible after comparing its situation with the point it occupied at some por- 
tion of time previous. As the nearest fixed stars are situated at such immense distances 
from us, it requires many years of observation to detect any motion in them. 

Q. In travelling through a forest, what apparent changes take place in the 
positions of the trees ? 

A. Those situated in the direction of our path seem to diverge 
or spread asunder^ while those which we are leaving behind us 
appear to be contracting or approaching each other. 

Q. What INDUCED Sir William Herschel to embrace the opinion of our solar 
system having a proper motion in space ? 

A. He observed a decided tendency in the stars in one part of 
the heavens to diverge or separate, and those in the opposite direc- 
tion to draw together or contract. 

Q. What did he infer from this appearance? 

A. He inferred from this that we are approaching the stars 
which appear to diverge, and receding from those which appear 
to draw closer together. 

The accompanying figure represents the effects of the Sun's motion in space. Let P 
be the point towards which we are approaching, and it will be perceived by the indica- 
tion of the arrows that the stars are separating or diverging in the Quarter towards 
which we are advancing. 

Fig. 114. 

i 



/ I \ 

Q. What APPEARANCE does a cluster of objects present, when receding 
from it? 

A. The individuals forming the cluster appear to be more 
crowded together the farther we recede from them. 

Q. Does the appearance of crowding or converging occur among the stars ? 

A. It does ; those stars in the opposite part of the heavens 
from which they seem to diverge, appear closer together. 



174 bouvier's familiar astronomy. 

Fig. 115. 



* 


\ 




*^ 


Q 


^* 


*** * 


© 


-«— t^ 


^^ 




^* 


/ 


\ 


\ 


•# 


♦ 



Let Q {fig. 115) be the point opposite to 
T?,fig. 114. It will be seen by the direction 
^ of the arrows that the stars are converging 
or crowding together. 



DIVISION VI. — parallax. 

Q. What is parallax ? 

A, Parallax is that arc of the heavens intercepted between the 
true and apparent place of a celestial body. 

Q. What is the parallax of the Sun or Moon ? 

A. The parallax of the Sun, Moon, or a planet, is the measure 
of the Earth's radius or semi-diameter, as seen from that body. 

Q. Is the calculation of parallax of any use in Astronomy ? 

A. The computation of parallax affords the means of ascertain- 
ing the distances of the heavenly bodies. 

Q. How can the distance of the Moon be ascertained? 

A. The distance of the Moon may be obtained by noting the 
instant of her touching the horizon^ and by drawing imaginary 
lines from her centre, one to the centre of the Earth, and the 
other to the eye of the spectator. 

Q. How can these lines, the one drawn to the centre of the Earth, and the 
other to the spectator, be measured ? 

A. The two imaginary lines, together with a line drawn from 
the spectator to the centre of the Earth, would form a right-angled 
triangle^ provided the Moon was in the horizon at the time of 
observation ; and as the distance from the surface to the centre of 
the Earth is known, as well as all the angles, the distance of the 
Moon from the centre of the Earth may easily be computed. 

Q. Where should a body be situated for the effect of parallax to be at the 
maximum ? 

A. The effect of parallax is always greatest when the body is 
situated in the horizon. 

Q. When a body is situated in the horizon, what is the parallax called ? 

A. It is called the horizontal parallax. 

Q. Have the fixed stars any parallax ? 

A Very few of the fixed stars have any sensible parallax. 



THE FIXED STARS. 175 

Q. Why have not the stars any parallax, as well as the Sun, Moon, and 
planets ? 

A. Because the stars are at such inconceivahly immense dis- 
tances from us, that with but few exceptions, we have no means 
of ascertaining their parallax. 

Q. Does the distance of a body have any eflFect on its parallax ? 

A. Undoubtedly ; the nearer a body is to the Earth, the greater 
is its parallax. 

Fig. 116. 




Suppose, in/.7. 116, A to be the station of an observer at any place on the surface of 
our globe, and E its centre. A planet at S will appear to the spectator at A, in the 
direction A C, while if it could be viewed from the centre of the Earth, its direction 
would be in the line E B. It is therefore seen from A at a point in the heavens below 
its position in reference to E. The angles formed by the intersection of the lines A S 
and E S is called the parallax of the planet. 

Q. What EFFECT does parallax produce upon the apparent places of the Sun, 
Moon, and planets ? 

A. Parallax causes the Sun, Moon, and planets to appear nearer 
to the horizon; that is, below the positions they would occupy if 
viewed from the Earth's centre. 

Q. Which planet would have the greatest parallax, Venus or Uranus ? 

A. Venus, because she is nearer to the Earth than Uranus ; 
and the nearer a body is to the Earth, the greater is its parallax. 

If, in the above figure, (116,) the point S had been placed as near again to A as it 
is there given, the lines A S and E S would be much more inclined than they are now 
drawn ; and, on the contrary, if S were removed to twice the distance from A that it is 
represented in the figure, the two lines would be less oblique to each other than they 
are now. 

Q. Does the effect of parallax upon the Sun, Moon, or a planet vary accord- 
ing to its POSITION with respect to the observer ? 

A. Yes ; if the body be in the horizon, the effect is greatest, 
and is then called the horizontal parallax. [See Note 47.) 

If the Sun, Moon, or a planet be at F, {fig. 116.) which is the horizon of the observer 
at A, the parallax is the greatest, and is called the horizontal parallax, which is the 
measure of the Earth's semi-diameter A E, as seen from the body at F. 



I 



176 



BOUVIER S FAMILIAR ASTRONOMY. 



Q. If a body be situated in the zenith, has it any parallax ? 
A. It has not. 

If a body were situated near the zenith, as at D, [fig. 116,) the parallax would bo 
very small, the lines A D and E D nearly coinciding with each other. A body situated 
in the zenith has no parallax. 

Q. If a star be observed from two opposite points of the Earth, would it ex- 
hibit no appreciable change of position ? 

A, A star exhibits no 'perceptible change of place from what- 
ever part of the Earth it may be viewed. 

Q. How, then, has any parallax been discovered in the fixed stars, if the dia- 
meter of the Earth is too small to subtend an angle at such immense distances ? 

A. Instead of using the Earth's diameter, of about eight thou- 
sand miles, as a measuring line, astronomers make use of the dia- 
meter of the Earth's orbit, which is about one hundred and ninety 
millions of miles. 

Q. How is the diameter of the Earth's orbit known ? 

A. It is known that the Earth is situated about ninety-five 
millions of miles from the Sun ; therefore, allowing the Sun to be 
placed in the centre of her orbit, the distance from one extremity 
of the orbit to the other would measure twice ninety-five millions, 
or, in round numbers, one hundred and ninety millions of miles. 
Fig. 117. 




The radius, which is the distance from 
the Sun to the Earth E, is ninety-five mil- 
lions j consequently the whole diameter 
from E to B is one hundred and ninety 
E millions. In the figure, the Sun is repre- 
sented in the centre of a circular orbit; but 
the Earth's orbit is slightly elliptical, the 
Sun being situated in one of the foci of the 
ellipse. 



Q. How can the Earth's orbit be used as a base line whereby to find the 
parallax ? 

A. If the place of a star be accurately observed on a given 
day, — the 1st of January, for instance, — and again in six months 
after, or on the 1st of July, its place be noted, and it be found 
that the diameter of the Earth's orbit subtends an angle, however 
minute, at that star, its distance could be computed as before 
stated. 

Q. What is the angle called which is formed by two converging lines drawn 
from opposite points in the Earth's orbit to a given star? 

A, The angle formed by two imaginary lines running from op- 



DISTANCES OF THE FIXED STARS. 177 

posite points of the Earth's orbit to a star, is called the parallactic 
angle. 

As regards most of the fixed stars, their distance from us is so immense, that although 
we view them in July and January from points one hundred and ninety millions of 
miles asunder, their apparent place is not changed; or, in other words, lines drawn from 
the extremities of the great base line of one hundred and ninety millions of miles to the 
star, would so nearly coincide, as to form no appreciable angle. 

Q. What is the parallax called which is due to the yearly motion of the 
Earth round the Sun ? 

A. It is called the annual parallax. 



CHAPTER II. 

iistantd 0f \\t im\ Stars. 

" How distant some of the nocturnal suns ! 
So distant, says tlie sage, 'twere not absurd 
. To doubt, if beams set out at Nature's birth, 
Are yet arrived at this so foreign -world; 
Though nothing half so rap^<i as their flight " — Touno. 

Q. Are the distances of any of the fixed stars accurately known ? 

A. But few of the stars have been found to have any sensible 
parallax; consequently, their distances are not certainly knotun. 

Q. At what DISTANCE are the fixed stars supposed to be from us ? 

A. The nearest of the fixed stars is supposed to be about 
twenty billions of miles from us. 

The ray of light by which we see the nearest of the fixed stars has been more than 
three years on its journey before it reaches our eye. 

Q. Has the distanck of any of the more remote stars been computed ? 

A. Yes ; Sir William Herschel calculated that some of the 
stars require a period of thirteen thousand years for light to 
travel from them to us at the rate of 200,000 miles in a second. 
{See Note 48.) 

Q. Have any other stars been discovered to have a parallax ? that is, have 
their distances been measured ? 

A. Yes ; a star in the southern hemisphere, in the constell.ition 
Centaur, called a Centauri, is supposed to be the nearest fixed 
star. Its distance is calculated to be twenty billions of miles. 

Q. How can such immense distances be measured ? 

A. By observing their different positions when viewed from 
stations widely asunder. 

Q. Does the same star appear to hold a different position when viewed from 
opposite sides of the Earth ? 

A. It does not. The Earth's diameter of eight thousand miles 
is too sinall to afford sufficient base for ascertaining the distance 
of the fixed stars. 

12 



178 



couvier's familiar astronomy, 



Q. How does a fixed star appear when viewed from opposite sides of the 
Earth ? 

A. It seems to hold the same position exactly. 

Q. From what two stations, more widely asunder than the opposite sides of 
the Earth, can any fixed star be viewed ? 

A. The two stations from which a fixed star may be viewed, 
are the opposite sides of the Earth's vast orbit. 

F\s 118. 



Let S (Jtg. 118) represent a fixed star, and a h the orbit of the Earth. 
If the Earth is at the point a on the 1st of January, she will be at the 
point b on the 1st of July. The distance between the two stations is one 
hundred and ninety millions of miles. If any fixed star be observed on 
the 1st of January on the line a S, on the 1st of July the same star will 
appear in the line b S, if the star have any parallax. 

Now, if instead of the star being drawn two inches from the Earth's 
orbit, let it be drawn at the distance of ten miles, preserving the orbit of 
the Earth of the same size as in the figure. It is evident that the dia- 
meter a b would be immeasurable from a point situated at that immerse 
distance. 



Q. How is it possible to ascertain whether a star seems to hold exactly the 
SAME POSITION in the heavens before and after an interval of six months ? 

A. Some small star is selected near to the one whose pa- 
rallax we wish to observe, and the distance of the object of 
observation from the small star is correctly ascertained at the 
two periods. 

Q. How is the distance separating the two stars determined ? 

A. This distance is determined by means of an instrument 
called a micrometer. 

The micrometer consists of parallel threads, which can be moved, by means of a 
screw, in such a manner that the threads may be brought close together or separated. 
The observer adjusts his micrometer so as to bring one of these threads over each star; 
then the number of turns of the screw is noted which is required to make the two threads 
meet. The screw for this purpose is very delicately cut. This mode of estimation as- 
sumes that the smaller of the two stars is so much more remote than its companion, as 
to have its apparent position affected much less by the observer's change of place. 

Q. What is this difi'erence of position a star seems to hold, accordingly as it is 
seen from one or the other side of the Earth's orbit, called ? 

A. This difference of apparent position of a star in the heavens, 
as seen from opposite sides of the Earth's orbit, is called its 
annual parallax. 



DISTANCES OF THE FIXED STARS. 179 

Q. How is this amount of parallax measured ? 

A. It is measured by what is termed the parallactic angle. 

The angle formed between the two converging lines drawn from the star S, {fig. 118,) 
one to a, and the other to 6, on the Earth's orbit. The angle S is the parallactic angle 
for the change of place from a to 6. 

Q. Have many of the fixed stars been found -within a measurable distance ? 

A, But few have been ascertained to be near enough to our 
system to admit of their distances being measured. 

According to the computations of Sir William Herschel, there are stars so distant, 
that light, which travels at the rate of 200,000 miles in a second, would require 13,000 
years to travel from those stars to our Earth. Hence it follows, that if such a star were 
to be at this moment extinguished, the inhabitants of the Earth would continue to see 
the star for 13,000 years to come ! 

Such contemplations are sufficient to overwhelm us. The question naturally arises. 
For what purpose were all these luminaries created ? '' Surely," says Sir William 
Herschel, " not to illumine our nights, which an additional moon of the thousandth 
part the size of our own would do much better ; nor to sparkle as a pageant, void of 
meaning and reality, and bewilder us among vain conjectures. Useful, it is true, they 
are to man as points of exact and permanent reference j but he must have studied astro- 
nomy to little purpose who can suppose man to be the only object of his Creator's care, 
or who does not see, in the vast and wonderful apparatus around us, provision for other 
races of animated beings." 

" Seest thou those orbs that numerous roll above? 
Those lamps that nightly greet thy visual powers 
Are each a bright capacious sun like ours. \ 

The telescopic tube -will still descry 
Myriads behind, that 'scape the naked eye ; 
And farther on, a new discovery trace 
Through the deep regions of encompassed space. 
If each bright star so m.any suns are found. 
With planecary systems circled round. 
What vast infinitude of worlds may grace. 
What beings people the stupendous space ! ! 
Whatever race possess the ethereal plain. 
What orbs they people, or what ranks maintain, 
Though the deep secret Heaven conceal below, 
One truth of universal scope we know : — 
Our nobler part, the same ethereal mind. 
Relates our Earth to all their reasoning kind; 
One Deity, one sole-creating Cause, 
Our active care and joint devotion drawe." 



180 BOUVIEPw'S FAMILIAR ASTROXOMY. 

COAPTER ni. 

Zhj glilkj Win, 

" Througlicut the Galaxy's exteniei line, 
Unnum.'bered orbs in gay confusicn shire 
TTliere every star that gilds the sl^rm z: -i^zt, 
TVith the fainu tremhling of a iista-- lirh:. 
Perhaps illumes scree systeri cf its c— n 
"with the strcng iiif.;ieiice of a raiiant 5 :,n." — ZirzjL:BSTH Cab ':=:?_ 

Q. Are there obv stars moke eemote than those -which are distinguishable by 
the naked eye ? 

A. There are innumeralle stars "^hich are not visible to the 
naked eve. 

Q. How is it known that there are stars which are betoxd tee beach of ur.- 
assisted vision ? 

A. As soon as the telescope is employed, numerous stars ap- 
pear which before were invisible. 

Q. How many additional stars does the telescope reveal ? 

A. Some parts of the heavens are found to be studded with a 

countless host of stars. 

Q. How MAXT staks may be seen at one time in the telescope ? that is. in one 
single field of view ? 

A. As many as five hundred have been seen in a single field 
of a telescope. 

Struve estimates that at least twenty millions of stat^ may be visible throughout the 
heavens in a good telescope. Sir William Herscbel once counted /j/1fy thousand stars in 
a space fifteen degrees long and two degrees wide. 

Q. Is an EQU-AJL xumber of telescopic stars visible in the different regions of 
the heavens ? 

A. By no means : in some directions hundreds are seen in the 
field of the telescope; whereas hxitfeiv appear if the telescope be 
turned to other portions of the heavens. 

Q. Can any oxe section of the heavens be designated as containing an un- 
usual number of stars ? 

A. There is o?ie great circle entirely surrounding the heavens, 
in which telescopic stars are very densely crowded together. 

Q. What is that great circle of stars called ? 

A, It is called the Milky Way. 

It is also known by the names of the Galaxy and Via Lactea. 
Q, What is the appearance of the Milky Way ? 

A. The Milky Way, as seen on dark nights by the naked eye. 
appears like an irregular stream offaint^ cloudy light. 

••A "crcai ani ample rcai, -whose dust is gcli 
Ani paveraent stars, as stars to thee appear. 
Seen in the Galaxy — ^that l^Iilky VTaj 
Which nightly, as a circling zone, thou sees:; 
FoT^ierei Trith stars."— Unroy. 



THE MILKY WAY. 



181 



Q Does the Milky Way form a circle in the heavens? 

A. It does : beginning not far from the north pole, and run- 
ning south, it divides into two branches near the ecliptic, after 
which thej reunite, pass near the south pole, and curve towards 
the north, extending in that direction to the place of beginning. 

Its course may easily be traced on a celestial globe or map. 

Fig. 119. 
Q. What is this milky light supposed to be ? 

A. It really emanates from thousands 
of stars which are too distant to be seen, 
but whose united light has this cloudy 
appearance. 

Q. Why do we see the greatest abundance of 
telescopic stars along this circular tract of the 
heavens called the Milky Way ? 

A. Because the group of stars in 
which our Sun is placed extends at a 
greater distance round us in that ring. 

Q. What is the supposed form of the Milky Way ? 

A. It is supposed to be in the form 
of a thin layer or stratum, comprised 
between two plane surfaces parallel and 
near to each other, but prolonged to im- 
mense distances in every direction. 

Sir John Herschel is of opinion that the Milky 
Way is not a stratum, but an annulus, or ring. 

Q. Where is our Sun situated with regard to 
the stratum called the Milky Way ? 

A. Our Sun is supposed to be one of 
the stars composing the Milky Wag, 
and to occupy a point very near the 
cefitre of it. 

In Jig. 119, the Galaxy or Milky Way is repre- 
sented according to the theory of Sir William Herschel. 
S represents the Sun in the centre. It will appear 
plain that a telescope pointed in the direction S A, 
S B, or S C, will reveal more stars than when pointed 
in the direction S D, S E, or S F, which accounts for 
some portions of the heavens being much more closely 
studded with stars than others. {See Note 49.) 

"How is night's sa"ble raantle labored o'er! 
How richly wrough-t with attributes divine! 
What wisdom shines! what love! this midnight pom.p, 
This gorgeous arch with golden worlds inlaid. 
Built with Divine A'mbition." — Touno. 




182 bouvier's familiar astronomy. 

CHAPTER IV. 

glagnituie d t^e <§tars. 

Q. When tlie telescope is directed to a star, is the same effect produced as 
"vrhen it is turned to a planet ? 

A. It is not. Generallv speaking, the planets present a circu- 
lar disc, whereas the stars appear like brilliant points. 

Q. Is any other difference observable between stars and planets when viewed 
through a telescope ? 

A. Yes. The telescope will magnify the discs of the planets, 
making them appear many times larger than when viewed with 
the naked eye ; but instead of magnifying the diameters of stars, 
it makes them appear smaller, but much brighter. 

Q. How can the telescope which magnifies the planets make the fixed stars 
appear smaller ? 

A. When we look at a star there is an optical illusion pro- 
duced, which makes it appear as if surrounded by rays ; the 
telescope divests it of this illusive radiation, and presents it to 
the eye as merely a brilliant point. 

Q. Would telescopes of higher magnifjing power make a star look larger 
than a point ? 

A. No ; with the highest magnifying powers the stars have no 
apparent magnitude. 

Q. Have we any other proofs that the stars have no sensible discs ? 

A, Yes ; when the Moon passes between us and a star, pro- 
ducing an occultation, the star is instantaneously hidden by the 
Moon's disc. 

Q. In case of an occultation by the Moon, what effect would be produced if 
the stars had apparent diameters ? 

A. The edge of the Moon would slowly cover them ; instead of 
which they always disappear instantaneously, preserving all their 
lustre until the moment of contact with the edge of the Moon. 

Q. If, then, all the fixed stars are without perceptible magnitudes, how can 
any idea be formed of their dimensions ? 

A. An idea of their magnitudes is formed by a measurement 
of their brilliancy. 

Q. How is the comparative brilliancy of the stars measured ? 

A. By means of an instrument called a photometer. 

A photometer designates the comparative brilliancy of any two luminous bodies, as 
that of a common candle and a gas-light, or of a candle and the noonday sun. 

Q. How much nearer to us would a telescope magnifying one hundred times 
oring a celestial body ? 

A. It would make a celestial body appear one hundred times 
nearer. * 

At the distance of the nearest fixed star, the whole orbit of our Earth, which is one 
hundred and ninety millions of miles in diameter, would appear but as a point. 



, MAGNITUDE OF THE STARS. 188 

Q. Has any comparison ever been instituted between the light of the Su^x and 
any of the fixed stars ? 

A. Yes. By means of the photometer it has been found that 
the light of Sirius, the brightest fixed star, is twenty thousand 
million times less than the light of our Sun. 

Q. Hovr FAR from us should our Sun be removed to appear no larger than the 
star Sirius ? 

A. It should be removed one hundred and fifty thousand times 
farther off. 

In speaking of Sirius, the poet says — 

" 'Tis strongly credited this owns a light 
And runs a course not than the Sun's less bright ; 
But that removed froioa sight so great a way. 
It seems to cast a dim and weaiker ray." — Sir Edward Sherburne. 

Q. Is Sirius one hundred and fifty thousand times more distant than oar 
Sun? 

A. Sirius is supposed to be at the least six hundred thousand 
times the distance of the Earth from the Sun. 

Q. What, then, is the estimated size of the sphere of the star Sirius ? 

A. It is conclusively proved that it is at least equal in magni- 
tude to fourteen of our suns. 

Dr. Wollaston, who made this estimate of the magnitude of Sirius, was not aware of 
the immense distance of that star from our system. Since that time, by observations 
made by Professor Bessel, it is proved that Sirius must be at least six hundred thou- 
sand times farther from us than our Sun, which would be at a distance beyond the power 
of human conception. If a railway car were to travel night and day at the rate of 
twenty miles an hour, it would require more than three hundred millions of years to 
accomplish the journey ! 

" Hail, naighty Sirius, m.onarch of the suns ! 
May we in this poor planet speak with thee ? 
Say, art thou nearer to His throne, whose nod 
Doth govern all things? — Hast thou heard 
One whisper through the open gate of heaven. 
When the pale stars shall fall, and yon blue vault 
Be as a shrivelled scroll?" — Sigournhy. 

Q. How does a bright star like Sirius appear through a powerful telescope ? 

A. When directed to the region of the heavens near the star, 
the light is so strong that it has the appearance of sunrise; and 
when the star is in the field of vision, the splendor is so great as 
to require a colored glass to protect the eye. 



184 bouvier's familiar astronomy. 

CHAPTER V. 

Ilptarance ai tlje Stars. 

"There they stand. 
Shining in order, like a living hymn 
Written in light." — N. P. Willis. 

Q. When the stars are examined through the telescope, have they the same 
APPEARANCE as when viewed by the naked eye ? 

A. They have not; when viewed through the telescope, they 
appear much more brilliant, and sometimes exhibit different colore, 
such as yellow, orange, red, green, &c. 

It is supposed that the color of Sirius, or the Dog star, has changed since it was first 
observed by astronomers. Mr. Barker considers that it has changed from red to white 
(its present color) in the lapse of ages, and quotes Aratus, Cicero, Virgil, Ovid, Seneca, 
Horace, and Ptolemy to sustain him. 

Q. Does the telescope reveal any other peculiarities with regard to the fixed 
stars ? 

A. It shows some stars to be variable and periodic^ others to 
be only temporary^ some compound, and others arranged in clus- 
ters and nehuloe. 

SECTION I. 

l^ariaWe anir ^eriobic Stars. 

" That man who has never looked up, with serious attention, to the motions and 
arrangements of the heavenly orbs, must be inspired with but a slender degree of reve- 
rence for the Almighty Creator, and devoid of taste for enjoying the beautiful and the 
sublime." — Dr. Dick. 

Q. Do all the fixed stars shine with the same degree of brilliancy? 

A. No ; many of the stars are known to vanish and reappear 
at regular intervals. 

Q. What are those stars denominated which vanish and reappear periodically ? 

A, They are called variable or periodic stars. 

Q. Do all the variable stars decrease in brilliancy till they disappear alto- 
gether, and then return again to view ? 

A. No ; some of the variable stars shining with the brilliancy of 
stars of the second or third magnitude, fade away until they ap- 
pear of the fifth or sixth, and then increase in brightness again 
till they attain their maximum splendor. 

Q. Do some become entirely invisible, and then appear again ? 

A. Yes ; some decrease in brilliancy till they become so faint 
as to be invisible with the best telescopes, after which thev appear 
again with their former brightness. 

Q. Which is the most remarkable periodic star ? 

A. A star in the neck of the Whale, known by astronomers 
as 3fira, or o (Omikron) Ceti; it decreases from a star of the 
second magnitude until it becomes invisible, and then returns to 
its former brightness. 



APPEARANCE OF THE STARS. 186 

Q. How LONG is the star Mira in passing through these variations of brilliancy ? 

A. It passes through these variations in about three hundred 
and thirty-one days. 

Q. Are there any other remarkable variable stars? 

A. There is a very remarkable star called Algol, in the con- 
stellation Caput MeduscE, 

The Constellation Caput Medusae is generally united with the constellation Perseus. 
The astronomical designation of Algol is /? (Beta) Persei. 

Q. How does the star Algol vary ? , 

A, It varies from a star of the second magnitude or brilliancy, 
to the fourth magnitude. 

Q. How LONG a PERIOD does Algol require to perform these variations ? 

A. Algol performs these variations in a little less than three days. 

The period of this star is gradually diminishing at the rate of about the tenth of a 
second in a year. Its period is now about 2d. 20A. 48/n. 53*37«. 

6 (Delta) Cephei, (S (Beta) Lyra, and some others, are known to be variable stars, as 
their periods have been estimated. But there are as many as fifty others whose degrees 
of variation and period are not determined with any degree of accuracy. 

Q. What is the periodic variation of the light of the stars supposed to indicate ? 

A. It is supposed to indicate either that these stars revolve on 
their axes, or that large opaque bodies revolve around them. 

Q. How can the variation of light be attributed to their revolution on their 
axes ? 

A. If luminous globes having opposite hemispheres of unequal 
degrees of brilliancy revolve upon their axes, the light and dark 
hemispheres must be periodically turned towards the Earth. 

Q. How could large opaque bodies revolving around them make their light 
variable ? 

A. If large opaque bodies, in the process of revolution round 
luminous globes, come between those luminous globes and the ob- 
server's eye at certain fixed intervals, some part of the light 
emitted from those globes must be periodically shut from our 
sight, or in other words, the luminous body must be eclipsed. 



SECTION II. 

"Oh! wh-O can lift above a careless look, 

While such bright scenes as these his thoughts engage, 
And doubt, while reading from so fair a book, 

That God's own finger traced the glowing page ; 
Or deem the radiance of yon blue expanse, 
With all its starry hosts, the careless work of Chance!" 

Mrs. Amelia B. Wslbt. 
Q. Have any changes ever been observed in the fixed stars ? 
A. Yes; some stars have entirely disappeared, while new ones 
are sometimes found which never ivere seen before. 



186 bouvier's familiar astronomy. 

Q. How can this disappearance of known stars and appearance of new ones 
be ACCOUNTED for ? 

A. Astronomers do not pretend to give any reason for it, the 
fact being founded entirely on observation. Some suppose it to 
be owing to dark orbs revolving round these luminous bodies; but 
this is only conjecture. 

"A million torches lighted hy Thy hand 

Wander unwearied through the blue ahyss , 
They own Thy power, accomplish Thy command,. 

All gay with life, all eloquent with bliss. 
"What shall we call them. ? Piles of ciy s tal light — 

A glorious company of golden streams — 
Lamps of celestial ether burning bright — 

Suns lighting other systems with their joyous beams ? 
But God, to these, is as the noon to night." 

Q. Have new stars appeared often in the heavens ? 

A. Yes, frequently. On one occasion, about A. D. 389, a new 
star shone out suddenly in the constellation Aquila, which was as 
bright as the planet Yenus. 

This star only remained visible three weeks, and then disappeared entirely. 
Q. How is it KNOWN when new stars appear ? 

A. There are catalogues of the principal stars, so that by con- 
sulting these, new stars are frequently detected. 

The appearance of a new star, b. c. 125, prompted Hipparchus to form a catalogue of 
all the principal stars, which was the first ever made. 

Q. Do these new stars appear with unusual brilliancy ? 

A. Not always ; but in 1572, Tycho Brahe, returning home 
one evening, saw a crowd of country people gazing at a star as 
bright as Sirius, which was not visible half an hour before. 

This star increased in brilliancy till it surpassed Jupiter in splendor. (A^ee Note 50.; 
Q. Are any of these temporary stars suspected of appearing periodically ? 

A. Yes ; the star of 1572, mentioned by Tycho Brahe, it is sup 
posed might have been the same as the bright star which appeared 
in the year 1264 ; and both these are also suspected as being 
identical with one which was seen in 945. 

If the star of the year- 945, that of 1264, and the star of 1572 could be proved to be 
one and the same, it could not be ranked among the temporary stars, but should more 
properly be classed as a variable star. 

Q. Can it be fully proved that the stars which have appeared once, and then 
have vanished, will ever be seen again ? 

A. No ; it would require centuries of close observation to ac- 
quire any just conception of the motions of bodies so distant and 
so vast. 

Many of the stars of the old catalogues are now missing, but some of these may have 
been the Asteroids, Uranus, or Neptune, in different parts of their respective orbits. 
Neptune, for instance, was recorded as a star, but has since proved to be a planet. No 
doubt some of the Asteroids and Uranus have also been catalogued as fixed stars. 



APPEARANCE OF THE STARS. 187 

SECTION III. 

Compottiib ^tars. 

"Take the glass 
And search, the skies. The opening skies pour down 
Upon your gaze thick showers of sparkling fire ; 
Stars, crowded, thronged, in regions so remote. 
That their swift beams — the swiftest things that be — 
Have travelled centuries on their flight to Earth. 
Earth, Sun, and nearer constellations ! what 
Are ye amid this infinite extent 
And m,ultitude of God's most infinite works." — Henh,t Ware, Jr. 

Q. How are the fixed stars supported in space ? 

A. By the combined influence of motion and attraction. 

The fixed stars are upheld in space by the operation of the same powers that serve to 
support the planets. 

Q. How can it be known that the fixed stars are subjected to the influence of 
motion and attraction ? 

A. Stars have been discovered, by the aid of telescopes, which 
have a revolution round each other in elliptical orbits. 

Q. What does the discovery of this elliptical motion of some stars round a com- 
mon centre of gravity tend to prove ? 

A. It tends to prove that gravitation is not peculiar to our 
system of planets and satellites, but that systems of suns in the 
distant realms of space are also subservient to its laws. 

DIVISION I. — DOUBLE STARS. 

" One star differeth from another star in glory." 

Q. When two adjacent stars are seen to revolve round a common centre of 
gravity, are they always of the same apparent brilliancy ? 

A, No ; the smaller or less brilliant one is seen to revolve round 
the larger star as its satellite. 

Q. What are these stars which revolve round each other called ? 

A. They are called double stars, or binary si/stems. 

Q. How do the two stars which revolve round each other appear ? 

A. When visible to the naked eye, the two stars appear as one. 

Some stars appear single, unless viewed through very powerful telescopes, when they 
may sometimes be separated into two or more. 

Q. What is the form of the orbits of stars composing a binary system ? 

A. The orbits are often veri/ eccentric ellipses. 

Q. Are the periods in which any of the double stars complete their elliptic 
revolutions known ? 

A. The periods of the revolutions of many of the double stars 
have been ascertained. 

The stars of I (Zeta) Herculis complete their revolutions in about 35 years : those of 
y (Gamma) Virginis, in 150 years ; those of 61 Cygni, in about 500 years ; and the double 
stars 65 Piscium require at least 3000 years to complete their revolutions. 



188 



BOUVIER S FAMILIAR ASTRONOMY. 



Fig. 120. Q. Have many of the fixed stars been found to be double ? 
A. Professor Struve has discovered more than 

■^■■^H two thousand double stars. 

HpSHj^H Q. Were they all visible to the naked eye ? 

&^3^H A. . They were not ; some can only be seen by 

^^^^^H means of the hest telescopes. 

^^^^^^^1 Q. Do all the stars which appear double belong to the same 

^^^^^^^1 SYSTEM? that is, have they a revolution round a common 

^^^^^^^H centre of gravity ? 

^^^^^H A. No ; the double stars are divided into two 

^^^^^H kinds, the optically double and i\\Q physically double. 

^^^^^^^H Q. What are the optically double stars ? 

^^^^^H A. Those stars which are not adjacent^ or united 

^^^^^H in one system, but are in fact very distant from 

^^^^^H each other, and appear as double, but are not really 

^HH^I so. These are called optically double stars. 

^^^^■^^H When two stars are situated nearly in a line with each other, the 

^^^^^^^^H light emanating from them will follow the same line of direction. 

^^^^^^^^H Consequently they have an appearance of contiguity, when in reality 

^^^^^^^H they are not adjacent, or even included in the same system, but far 

^^^^^^^H removed from each other. Thus, in Jig. 120, the star a is nearer t<) 

^^^^^^^H the eye than the star b. but in the vast sphere of the heavens their 

^^^^^^^H distances appear the same ; and in consequence of the light emanac- 

^^^^^^^H ing from both a and b pursuing the same line of direction, they seem 

^^^^^^^^H to be contiguous, as shown at the top of the engraving, when actu- 

^^^^^^^^H ally they are very remote from each other. 

^^^^I^^l Q. What are physically double stars ? 

^^^^^H A. Physically double stars are those which com- 

^^^^^H pose one system, and are united by mutual gravi- 

^^^^^H tation; the smaller one revolving round the larger, 

^^^^^1 or more properly, about their common centre of 

Hjj^^^l gravity. 

BSu^^^^l Q. Are both stars in binary systems always of the same 

^^^^^^H color? 

^^^^^H A. Not always ; one star in a binary system is 

^H^^^H often of a different color from its companion. 

^^^^^^^^H When this is the case, the two stars have generally complimentary 

^^H^^^^l colors ; that is, those contrasted colors which, when united, form 

^^^^^^^H white light. Orange and blue, for instance^ or green and red^ are 

^^^^^^^H complimentary colors. 



The star y (Gamma) Virginis is a very remarkable specimen of a binary system. 
One star is supposed to revolve about the other in a period of about 180 years. The 
positions of the two stars at diflferent epochs may be seen by reference to^igr. 121. In tbe 
year 1836 they are in a line with the Earth, and therefore appear as a single star, the one 
being hidden behind the other. They may be seen separating from 1837 until the year 
1860, when they are shown to be again far asunder. These two stars are really a' ways 



APPEARANCE OF THE STARS. 



189 



at about the same distance from each other, but only appear single, owing to their situa 
tions with regard to us. 

Fig. 121. 




DIVISION TI. — MULTIPLE STARS. 

" And these are suns — vast central living fires — 
Lords of dependent systems — kings of worlds 
That wait as satellites upon their power, 
And flourish in their smile. Awake, my soul. 
And joaeditate the wonder ! Countless suns 
Blaze round thee, leading forth their countless worlds • 
"Worlds, in whose bosonas living things rejoice, 
And drink the bliss of being from the fount 
Of all-pervadmg Love !" 

Q. Are there any systems of stars known to contain more than two monibera 

A. Yes ; mauT/ systems contain more than two stars. 



190 bouvier's familiar astronomy. 

Q. What are these systems of stars containing more than two members 
called ? 

A. They are called multiple stars. 

Q. Have any stars been seen having more than three members ? 

A. Yes; besides triple, there are also quadruple stars, and 
assemblages of even five or six stars. 

The star a (Sigma) Orionis may be resolved into a " double triple star," or two sets 
of stars containing three stars in each set. The star (Phi) Orionis, when seen through 
a powerful telescope, may be resolved into six component stars. 

Q. Among the triple stars is there any instance known of a revolution 
round a centre ? 

A. Yes ; in the star ^ (Zeta) Cancri a powerful telescope shows 
three stars instead of a single one, two of which revolve about the 
third. 

SECTION IV. 

Clusters anb g^ebalae. 

•• To count tbeix numbers were to count the sands 
That ride in whirlwinds the parched Lybian air; 
Or waves that, when the blustering north embroils 
The Baltic, thunder on the German shore." 

Q. Are the stars regularly scattered over the firmament ? 

A. No : in some places they are crowded together ; in others, 
thinly dispersed, 

Q. What are these crowds of stars called ? 
A. They are called clusters. 

Q. What does this clustering of the stars seem to indicate ? 

A. That members of the group or cluster have an attraction 
for each other, and a peculiar tendency to group themselves 
together. 

Q. Are any clusters of stars visible to the naked eye ? 

A. Yes ; the well-known group called the Pleiades is a 
cluster. 

This cluster, when examined with a good telescope, is found to consist of a great 
number of stars. 

Q. What appearance has a cluster to the naked eye ? 

A. It usually presents a white or hazy light. 

Q. What is the reason many clusters of stars appear like milky-white 
spots ? 

A. Because they are situated at such immense distances from 
us, that their united light appears only like a faint cloud. 

Sir William Herschel calculated that a cluster consisting of five thousand stars, 
although three hundred thousand times more distant from us than Sirius, could be 
detected by the aid of a forty-feet telescope, as a milky spot of light. Lord Rosse's 
great telescope would, in all probability, catch a glimpse of them if removed to twice 
that distance. 



APPEARANCE OF THE STARS. 191 

Q. When viewed through a telescope, how do those distant clusters appear ? 

A, They then appear to be composed of countless numbers of 
Btars congregated together. 

In the constellation Hercules there is a globular cluster which is truly magnificent 
when seen through a powerful telescope. To the naked eye this cluster appears like a 
hazy object ; but when viewed through a powerful instrument, its aspect is grand be- 
yond description. The stars seem to be greatly condensed towards the centre, which 
gives forth a blaze of light. 

Q. What is the usual form of clusters ? 

A, Some are globular^ others irregular in their form. 

Sir John Herschel says, " Among the most beautiful objects of this class is that which 
surrounds the star k (Kappa) Crucis. It occupies a square area of about one minute and 
a quarter of arc, and consists of at least one hundred and ten stars, from the seventh 
magnitude downwards ; eight of the more conspicuous of which are colored with various 
shades of red, green, and blue, so as to give to the whole the appearance of a rich piece 
of jewelry." 

Q. Are those specks of cloudy light called clusters ? 
A, Yes ; unless, indeed, the best telescopes fail to show them 
to be composed of minute stars. 

Q. Has the number of stars in one of these faint clusters been computed ? 

A. Yes ; many clusters which appear like cloudy specks have 
been computed to contain ten or twenty thousand stars when 
viewed through good telescopes. 

If each of these stars be a sun, surrounded by a train of planets and comets, and each 
of these suns as far distant from each other as our Sun and the nearest fixed star, how 
vast must be that syst<5m, the combined light of whose thousands of suns appears only 
like a faint haze, and which has required thousands of years to reach us ! 

Q. What are these cloudy spots of light called which the best telescopes 
cannot resolve into stars ? 

A. They are called nebulce. 

Nebula means a mist or cloud. 
Q. Why are these specks of faint, cloudy light called nebula ? 

A. Because when the name was conferred on them, they were 
thought to be of a mistg or cloudy nature, like the substance of 
some of the comets. 

Q. How are the nebulae divided ? 

A. First, into nebulae properly so called, in which there is no 
appearance of stars ; secondly, planetarg nebulae ; thirdly, stellar 
nebulae, or nebulous stars. 

Q. Are there many nebulae to be seen in the heavens ? 

A. About two thousand nebulas and clusters were observed by 
Sir William Herschel. 

Their places were computed by Miss Caroline Herschel, the sister of Sir William, a 
lady eminent for her scientific knowledge and unwavering perseverance in astronomieai 
discovery. She arranged these nebulae and clusters in a catalogue iu order of their 
right ascension. 

Q. Do nebulae assume any regular form ? 

A. They do not; some aj^pear elliptical, some annular, some 
globular, kc. 



192 



BOUVIER S FAMILIAR ASTRONOMY. 
Fig. 122. 




The figure represents an elliptical nebulae in the constellation Andromeda, as seen by 
Mr. George P. Bond, in the great refractor, Cambridge, Massachusetts. 
Q. What are annular nebulge ? 

A. Annular nebulae are those which appear in the form of a 
ring. 

Fig. 123. 



The figure represents the annular nebula of the 
constellation Lyra, as seen through Lord Rosse'.s 
telescope. Through ordinary telescopes this nebula 
appears rather darker in the centre than at the 
edges, and looks like a hoop with gauze stretched 
over it. Bat the powerful telescopes of Lord Rosso 
resolve it into excessively minute stars, with a fringe 
of stars round the outer edge. 



Q. Can. GLOBULAR nebulge be easily resolved into stars ? 

A. Yes ; globular nebulae are generally more easili/ resolved 
into stars than elliptical nebulae. 

Q. What are planetary nebulse ? 

A. They are nebulae which have a close resemblance to planets, 
presenting round or oval discs, some of which are sharply defined, 
others a little hazy or softened at the edges. 

Q. Have many planetary nebulae been discovered ? 

A. No ; only about twenty-five or thirty, most of which are in 
the southern hemisphere. 




APPEARANCE OF THE STARS. 193 

Q. Do planetary nebulae emit much light ? 

A. Yes ; their light occasionally rivals that of the planets. 

Planetary nebulae are often attended by minute stars, which give the idea of accom^ 
panying satellites. 

Fig. 124. 




The above figure represents a planetary nebula, of a pale bluish white, situated m the 
coustellatiou Argo. This nebula is immediately adjacent to some small stars, which 
are shown in the figure. 

Q. Have the magnitudes of planetary nebulae been estimated ? 

A. Not with any degree of certainty^ as their distances are 
unknown to us. 

Sir John Herschel says, ** Granting these objects to be equally distant from us with 
the stars, their real dimensions must be such as would fill, on the loo:efit computation, 
the whole orbit of Uranus;" that is, they must be at least three thousand six hundred 
millions of miles in diameter. 

Q. What are stellar nebulae ? 

A. These are nebulae having a round or oval shape, increasing 
in density towards the centre. 

Sometimes the condensation is so great as to give the appearance of a star with a 
blurred outline, or like a candle shining through horn. 



DIVISION I. — DOUBLE NEBULA. 

" Could we wing our way to the highest apparent star, we should then see other skies 
expanded, other suns that distribute their inexhaustible beams of day, other stars that 
gild the alternate night, and other, perhaps nobler, systems established — established iu 
unknown profusion through the boundless regions of space. Nor do the dominions of 
the (jtReat Sovereign end there: even at the end of this vast tour, we find ourselves 
advanced no farther than the frontiers of creation, arrived only at the suburbs of the 
Great Jehovah's kingdom." — Hervev. 

Q. Are nebulae, like the fixed stars, ever found double ? 

A. Yes ; double nebulae occasionally occur, but they are not as 
common as double stars. 

Q. Have double uebulae any physical connection with each other ? 

A. It is not certainly knoivn that any relation subsists between 
them, yet the fact is strongly suspected. 

Q. Have any of the double nebulae been observed to have a revolution round a 

COMMON CENTRE ? 

A. They have not^ with any certainty ; jet it is believed they 
have a slight angular motion, which in the course of some hundreds 

18 



10-4 bouvier's familiar astronomy. 

of 3"ears may be sufficient to afford data for the computation of 
their orbits. 

Q. Are double nebuloe supposed to consist of stars ? 

A. Yes ; no doubt the greater number are composed of thou- 
sands of stars each resembling our Sun. 

Sir John Hersehel says, " Their stupendous scale, the multitude of individuals they 
involve, the perfect symmetry and regularity which many of them present, the utter 
disregard of complication in thus heaping together system upon system, and construc- 
tion upon construction, leave us lost in wonder and admiration at the evidence they 
afford of infinite power and unfathomable design." 

Fig. 125. 




F!;i. 125 represents a double nebula in the constellation Virgo, with two other nebulae 
in the field uf view at the same time. This double nebula may consist of stellar systems, 
each revolving round the other — each a universe. 

■•Stars teach, as well as shino." 



DIVISION II. — NUBECULA, OR MAGELLANIC CLOUDS. 

" I -wonder as I gaze. That stream of light, 
Undimraed, unquenched— just as I see it now — 
Has issued from those dazzling points, through j'earg 
That go back: far into eternity. 
Exhaustless flood ! forever spent, renewed 
Forever!" 

Q. What are the Magellanic clouds ? 

A. They are two nebulous masses of light, plainly visible to the 
naked eye. 

Q. Where are tbe Magellanic clouds situated? 
A. In the southern hemisphere, near the south pole. 
Q. What do they resemble ? 

A. They are like some of the brighter portions of the Milky 
Way. 



METEORS. 195 

Q. By what name are the Magellanic clouds known to astronomers ? 
A, They are called Nuheculce, The larger cloud is called 
Nubecula major^ and the smaller one, Nubecula minor. 
Q. Of what are the Magellanic clouds supposed to consist ? 

A. The Nubeculae or Magellanic clouds are supposed to con- 
sist of an immense number of nebulae ; the greater Nubecula 
contains nearly three hundred, and the lesser one neaxXy forty _, 
nebulae. 

The Nubecula major consists of upwards of 900 stars, nebulae, and clusters, and Nu- 
becula minor of more than 200. The Magellanic clouds, or, as they are generally 
termed by astronomers, Nubeculae, were known to the Arabian astronomers as El-Bakar, 
or the White Ox. These two nebulous clouds of light are of an oval form ; their bright- 
ness is such that the full moon obscures the light of the lesser, and renders the greater 
cloud barely visible. 

The Nubeculas never appear above the southern horizon of those who live farther 
north than from 16° to 20° north latitude. 



CHAPTER VI. 



Q. What are meteors ? 

A. Fiery or luminous bodies occasionally seen moving rapidly 
through the atmosphere. 

Q. Where have meteors their origin ? 

A, They are supposed to have their origin beyond our 
atmosphere. 

They are considered as belonging to a nebulous body with which our Earth come? 
periodically in contact** 

Q. Of what do meteors consist ? 

A. They are supposed to consist of light combustible matter, 
which moves with great velocity. 

Q. Are they ever of large dimensions ? 

A. Some have been seen which were several thousand feet in 
diameter. 

Q. What EFFECT is produced when these bodies enter our atmosphere? 

A. They condense the air before them so rapidly, that they 
elicit the heat which sets them on fire. 

Q. By what names are luminous meteors sometimes known ? 

A. They are sometimes, though very improperly, called shoot- 
ing stars. 

^. From WHAT are these meteors supposed to emanate ? 

A. They are supposed to descend to us from some nebulous 
body far beyond the regions of our atmosphere. 



* This theory of meteors and their origin is chiefly according to Professor Olmsted, 
who has devoted much attention to this subject. 



196 bouvier's familiar astronomy. 

Q. Do they iguite far above the sui-f^ace of the Earth ? 

A. Yes ; they sometimes take fire at the distance of thirty 
miles above our Earth. 

Q. Do meteors appear very frequently ? 

A. Yes ; at all times of the year ; but they are more frequentW 
seen about the month of JSovemher. 
' Q. How do meteors make their appearance ? 

A. They usually appear suddenly in the clear, azure sky, and 
darting with great rapidity, are extinguished without noise, 
sometimes leaving a film of smoky vaj^or to indicate the spot they 
occupied. 

• In the twenty-ninth volume of the Transactions of the Royal Society of London, 
there is an interesting article from the pen of the great astronomer, Edmund Halley, 
containing an account of some meteors seen by him in the year 1718. A very remark- 
able one is cited by Halley, on the testimony of Sir Hans Sloane, who saw it while 
walking the streets of London. This meteor appeared in the evening, a few minutes 
after eight o'clock, the sky being clear, and the Moon shining brightly, near the meri- 
dian. Suddenly a great light appeared in the west, which he at first attributed to 
rockets or fireworks; but he was soon undeceived, for on casting his eyes towards the 
light, he saw a splendid meteor in the direction of the Pleiades, having a long lumi- 
nous train or tail of a most dazzling brilliancy. It left behind it a track of a yellowish- 
red color, which seemed to sparkle. The splendor of this meteor was little inferior to 
that of tlie Sun ,: and so strong was the light, that within doors candles were of no use ; 
and although the Moon shone brightly, her light was scarcely visible. In fact, for a 
few seconds, the light resembled that of day. 

Q. What is the general direction of meteors ? 

A. They appear in all parts of the heavens, and dart in all 
possible directions ; but the greatest number move from east to 
west. 

Meteors sometimes fall to the Earth in showers of thousands, which emit so much 
light as to illuminate the heavens. A shower of this kind was seen in November, 1799,- 
another in November, 1831; also in November, 18.32, multitudes of shooting stars fell 
in the western part of Asia and southern part of Europe. But the most magnificent 
shower of meteors which has ever been known was that which fell during the night of 
November 12, 1833. This shower commenced at nine o'clock in the evening, and con- 
tinued till the morning sun concealed them from view. It extended from Canada tc 
the northern boundary of South America, and over a tract of nearly three thousand 
miles in width, its western limit extending to longitude 100° west from Greenwich. 



SECTION I. 

gletcoritcs or gierolites. 
Q. Do meteorites resemble meteors ? 

A. They frequently have the same brilliancy as meteors, and 
sometimes emit a much brighter light. 

Q. Are meteorites attended with any noise ? 

A. Yes; they are sometimes attended with a loud hiss'nj 
noise, and sometimes it resembles the report of a cannon. 



Q. Are meteorites tangible substaucej' 
A. They are. 



METEORS. 197 

Q. Do they ever fall to the Earth ? 

A. Yes; and they are composed generally of a few metallic 
elements, as iron and nickel, &c. 

Q. Do these metallic substances ever fall to. the Earth in large quantities? 

A. They do. Sometimes they fall in large masses, and some- 
times in showers of stones. 

In the year 1620 a violent explosion was heard at a village in India, and at the same 
time a luminous body fell to the Earth. The officer of the district immediately repaired 
to the spot to examine the cause of the phenomenon. He employed some men to dig, 
and as they threw up the earth he found the heat increase very perceptibly, till finally 
they reached a lump of iron excessively hot. This curiosity was sent to court, and the 
king had it weighed in his presence. He then ordered it to be made into a sabre, a 
knife, and a dagger, for his own use. But the workmen found, after several trial,-;, that 
the thing was impossible, as the metal was not malleable, but shivered under the ham- 
mer; they afterwards mixed with it a one-third part of common iron, when they were 
able to produce excellent blades. The historian of this king adds — " During his reign 
the Earth retained order and regularity; raw iron fell from lightning, which was, by 
his world-subduing authority, converted into a sabre, a knife, and a dagger." 

Q. Do meteorites ever pass neak to the Earth without billing to its surface ? 

A. Yes; sometimes they approach to within a short distance 
of the Earth without falling on its surface. 

Mrs. Somerville mentions one which passed within twenty-five miles of our planet, 
and which was estimated to weigh about six hundred thousand tons, and to move with 
the velocity of twenty miles in a second. Only a small fragment of this immense mass 
reached the Earth. 

Q. Why are meteorites supposed to be of foreign origin ? 

A. Because they have peculiar characteristics, which belong to 
no native rocks or stones with which we are acquainted. {See 
Note 51.) 

Q. Are there any other reasons for supposing they have not their origin on 
our Earth ? 

A. The almost invariable obliquity of their descent, and the 
explosion accompanying their fall, show they are not of terrestrial 
origin. 

Q. Have meteorites of large size ever fallen to the Earth ? 

A. Many weighing ^Yom fifty to a hundred jwunds have fallen 
to the Earth, and in some instances the masses have been much 
larger. 

Q. How, then, is the origin of meteorites accounted for ? 

A. They are supposed to be fragments of some body or bodies 
which are revolving round the Sun, and become visible when they 
are inflamed by entering our atmosphere. 

These bodies, which may exist by thousands in the vicinity of our globe, are supposed 
to revolve in orbits having a period corresponding to one or more revolutions of the 
Earth, which will account for our seeing more of them at certain seasons of the year. 
But all this is mere conjecture ; nothing is yet certainly known with regard to the.<€ 
wonderful appearances. 



198 bouvier's familiar astronomy. 

SECTION II. 

Q. What is the zodiacal light? 

A. A conical-shaped light which appears at certain seasons of 
the year just after sunset, or immediately before sunrise. The cen- 
tre of the base of the cone is in a line with the centre of the Sun. 

Q. What is the appearance of this light ? 

A. It resembles the light of the Milky Way. 

Q. What is this light supposed to be ? 

A, It is supposed to be a nebulosity around the Sun. 

Q. If this be the case, how would our Sun appear to the staks? 

A. He would appear as the nebulous stars do to us. 

Q. At what seasons of the year is the zodiacal light most plainly visible ? 

A. During twilight in northern latitudes, in the evenings of 
March, April, and May ; and in the morning twilight of the 
months of September, October, and Xovember. 

Q. What is the magnitude of the zodiacal light ? 

A. It extends beyond the orbits of Mercury and Venus, and 
perhaps reaches even as far as our Earth ; which would be equal 
to ninety -five millions of miles in length. 

Sir John Herschel thinks it may possibly be the denser part of the ethereal medium, 
which, it is believed, is the substance which resists the motions of comets. He also sup- 
poses this nebulous envelope may be composed of the materials of the tails of millions 
of comets, which have been stripped of these appendages during their perihelion pas- 
sage. Some have supposed this nebulous light to be the atmosphere of the Sun ; but 
modern astronomers are of opinion that the existence of a gaseous envelope of such 
enormous dimensions could not be sustained according to the laws of dynamics. 

Q. Is there any supposed relation between the zodiacal light and the meteoric 
showers ? 

A. Professor Olmsted, of Yale College, and Messrs. Arago and 
Biot, are of opinion that the substance composing the zodiacal 
light furnishes the materials which constitute the meteoric showers. 

{See Note 52.) 

So little is definitely known in this department, that most of it is merely conjecture. 
All we can do is to state the facts, and give the opinions which the most learned astro- 
nomers have deduced from them. 

'• That which we know,is little ; that which we know not,is immense." — La Place. 



CHAPTER Til. 

Constelbtions. 

To Teachers. — The configurations of the heavens are to the astronomer what the 
natural divisions of the Earth are to the geographer. To the student of geography, the 
natural divisions of the Earth, its oceans and continents, the situation of its poles, equa- 
tor, and tropics, are the first steps towards a more thorough acquaintance with the 
science. In like manner, it is necessary that the student of astronomy should have a 
correct idea of the relative positions of the principal groups of stars, the places of the 
poles, equinoctial and ecliptic, else he would be as much at a loss as the traveller with- 



CONSTELLATIONS. 



100 



out a landmark. It is desirable, therefore, that the following questions should be 
answered, without committing them to memory, but merely by the aid of the map or 
globe, in order that the student may become familiar with the constellations and their 
relative situations. The localities of those constellations of the southern hemisphere, 
which are never seen in the United States, should be learned, for the same reason that 
we study the geography of countries which we never expect to visit. 



The following list of zodiacal constellations in their order, and of the northern and 
southern constellations alphabetically arranged, with their astronomical names and 
translations, are given below. Table XL, at the end of the volume, shows them in order 
of right ascension, with their declinations, &c. 



ZODIACAL CONSTELLATIONS. 



Ns 



Aries 

Taurus 

Gemini 

Cancer 

Leo 

Virgo 

Libra 

Scorpio 

Sagittarius... 
Capricornus. 

Aquarius 

Pisces 



Ram. 

IJull. 

Twins. 

Crab. 

Lion. 

Virgin. 

Balance 

Scorpiop. 

Archer. 

Sea-goat. 

Water-bearer. 

Fishes. 



NORTHERN' CONSTELLATIONS. 



Name. 

Andromeda 

Antinous , 

Aquila 

Auriga , 

Bootes 

Camelopardalus 

Canes Venatici 

Caput Medusa 

Cassiopeia 

Cepheus 

Cerberus 

Coma Berenices 

Cor Caroli 

Corona Borealis 

Cygnus 

Delphinus 

Draco 

Equuleus 

Hercules 

Honores Frederici 

Lacerta 

Leo Minor 

Lyncis ....; 

Lyra 

Mous Menalus 

Musca Borealis 

Ophinchus 

Pegasus 

Perseus 

Quadraas Muraiis 

Sagitta 

Scutum Sobieski 

Serpens 

Tarandus 

Taurus Poniatowski... 
Telescopiumllerschelii 

Triangulum 

Triangulum Minus 

Ursa Major 

Ursa Minor 

Vulpecula 



Trauslatioii. 



.\ndrouieda. 

Antinous. 

i<]agle. 

Charioteer. 

Bear-driver. 

Cameleopard. 

Greyhounds. 

Medusa's Head. 

Cassiopeia. 

Cepheus. 

Three-headed Dog. 

Berenice's Hair. 

Charles's Heart. 

Northern Crown. 

Swan. 

Dolphin. 

Dragon. 

Little Horse. 

Hercules. 

Frederick's Glory. 

Lizard. 

Lesser Lion. 

Lynx. 

Lyre. 

Menalus Mountain. 

Northern Fly. 

Serpent-bearer. 

Flying Horse. 

Perseus. 

Mural Quadrant. 

Arrow. 

Sobieski's Shield. 

Serpent. 

Reindeer. 

Poniatowski's Bull. 

Herschel's Telesc pe. 

Triangle. 

Lesser Triangle. 

Great Bear. 

Little Bear. 

Fox. 



SOUTHERN CONSTELLATIONS. 



Antlia Pneumatica 

Apparatus Sculptoris., 

Apus 

Ara 

Argo Navis 

Avis Solitarius 

Canis Major 

Canis Minor 

Cela Sculptoria 

Centaurus 

Cetus 

Chameleon 

Circinus 

Columba 

Corona Australis 

Corvus 

Crater 

Crux 

Dorado 

Equuleus Pictorius 

Eridanus 

Felis 

Fornax Chemica 

Globus JCthereus 

Grus 

Horologium 

Hydra 

Hydrus 

Indus 

Lepus 

Lupus 

Machina Electrica 

Microscopium 

Monoceros 

Mons Mensae 

Musca Australis 

Norma 

Octans 

Officina Typographia... 

Orion.... 

Pavo 

Phoenix 

Pisces Australis 

Pisces Volans 

Psalterium Georgia- ) 

num. J 

Pyxis Nautica 

Reticulus 

Rober Caroli 

Sceptrum Braudeu- ) 

burgium J 

Sextans 

Solarium 

Telescopiura 

Triangulum .\ustralis. 
Toucaua 



Trauslution. 



Air-pump. 

Sculptor's Workshop. 

Bird of Paradise. 

Altar. 

Ship Argo. 

Owl. 

Great Dog. 

Little Dog. 

Graver's Tools. 

Centaur. 

Whale. 

Chameleon. 

Compasses. 

Dove. 

Southern Crown. 

Crow. 

Cup. 

Cross. 

Sword-fish. 

Painter's Easel. 

River Po. 

Cat. 

Chemical Furnace. 

Balloon. 

Crane. 

Clock. 

Water-serpent. 

Water-snake. 

Indian. 

Hare. 

Wolf. 

Electrical Machine. 

Microscope. 

Unicorn. 

Table Mountain. 

Southern Fly. 

Rule and Square. 

Octant. 

Printing Press. 

Oiion. 

Peacock. 

Phoenix. 

Southern Fish. 

Flying Fish. 

George's Harp. 

Mariner's Compiss. | 

Net. I 

Charles's Oak. | 

Sceptre of Brauden- ] 

burg. I 

Sextant. \ 

Suu-dial. - 
Telescope. 
Southern Trinngle. 

American Goose. ! 



200 bouvier's familiar astronomy. 

"Make frieniship -witli the stars." — Mks. Sigourust. 
Q. How are the heavens divided ? 
A. They are divided into groups or divisions, 

Q. "What are these gi'oups or divisions called ? 

A. They are called constellations, 

Q. Why are the heavens divided into groups or constellations ? 

A, Because it would be impossible to designate each star by a 
joarticular name; therefore it became necessary to separate them 
into groups or divisions, which are called constellations. 

Q. How are these constellations marked upon maps or globes ? 

A. They are made in the form of men, monsters, birds, 

fishes, ^0. 

Q. Why were they thus figured ? 

A, Because the ancients observed certain collections or groups 
of stars rose or set at certain seasons, for which they alwnys had 
a symbolical representation of some animal, bird, kc. They also 
placed their gods among the stars. 

Q. Were the stars grouped into constellations by the ancients ? 

A. Yes ; we read of the ancients being acquainted with Orion, 
the Pleiades, and the Great Bear, from the most remote antiquity. 

Q. Into how MANY constellations did the ancients divide the heavens ? 

A. Into forty -eight ; but the moderns have added fifty-eight to 
that number, making in all one hundred and six. 

Q. How were the forty-eight constellations of the ancients divided ? 

A. They were divided into twelve zodiacal constellations, twenty- 
one of the northern, and fifteen of the southern hemisphere. 

SECTION I. 

^obiatal Couskllatioits. 

Q. Have tne names of the zodiacal constellations been changed, or additions 
been made to them ? 

A. No ; they are the same, and retain the same names as those 
mentioned by Hipparchus two thousand years ago. {See Note 5o.) 

Q. Are the stars belonging to each constellation designated by particular 

NAMES ? 

A. Some of the principal stars have particular names assigned 
to them, as Regulus, Arcturus, Capella, &c. ; but besides these 
they have another designation. 

Q. What other designation have they ? 

A. They are designated by the letters of the Greek alphabet, 
Regulus is also known as a (Alpha) Leonis ; Alshain as ^ (Beta) 
Aquilse ; and Errai as y (Gamma) Cephei. 

But if there are more stars in any constellation than there are letters of the Greek 
alphabet, the Roman alphabet is then used, and after that the numbers 1, 2, 3, <fec. 
The first letter of the Greek alphabet usually denotes the brightest star in the constel- 
lation j ,J, (Beta,) the second; y, (Gamma,) the third, in brilliancy; and so on. 



CONSTELLATIOXS. 201 

It is not generally known that Sir William Herschel was so well acquainted with the 
ftiirs, that he could unhesitatingly call every one, down to the sixth magnitude, by its 
name and letter. 

Q. How are the constellations divtdkd ? 

A, Into the zodiacal constellations, and those of the northern 
and southern hemisphere. 

Q. What are the zodiacal constellations ? 

A, Those constellations which lie in that zone called the 
zodiac, 

Q. "What is the zodiac ? 

A. A belt or zone^ extending round the Earth, sixteen degrees 
in width, through the middle of which runs the ecliptic. 

Q. What are the names of the zodiacal constellations ? 

A. Their names and their signs are as follows : 

T Aries, the Ram. — Libra, the Balance. 



b' Taurus, the Bull. 
K Gemini, the Twins. 
@ Cancer, the Crab. 
Q, Leo, the Lion. 
y(jl Virgo, the Virgin. 



1U Scorpio, the Scorpion. 
7(^ Sagittarius, the Archer. 
P6 Capricornus, the Goat. 
— Aquarius, the Water-hearer, 
K Pisces, the Fi^ " 



ARIES. 



Q. Where may the constellation Aries be found upon a globe or map ? 

A. Aries, formerly the first constellation of the zodiac, now 
the first sign and second constellation, was situated at the vernal 
equinox ; but owing to precession, that constellation is now re- 
moved 30° westward. 

As the sidereal year is twenty minutes twenty seconds longer than the tropical year, 
the equinoctial points have a retrograde motion, called the precession of the equinoxes. 
About 2000 years ago the equinoctial points were fixed in the constellations Aries and 
Libra; but the precession equals about 50" annually, so that now the equinoctial points 
are 30°, or a whole sign, westward of the places first assigned to them ; consequently 
the sign T (Aries) is now in Pisces, and 8 (Taurus) in Aries, <fec. At this time the con- 
Etellation Pisces occupies the same place on the zodiac which Aries did 2000 years ago. 
Q. How is the constellation Aries known in the heavens ? 

A. It is known by two bright stars, about 4° apart, which are 
on the meridian at midnight on the 25th of October. 

The brighter of the two stars is called Arietis, or a Arietis ; the second, which lies a 
little to the south-west of Arietis, is called .8 Arietis, or Sheratan. These two stars aro 
in the head of Aries 

TAURUS. 

Q. How can Taurus be distinguished ? 

A. About the beginning of March the constellation Taurus 
may be seen in the evening to the east of Arietis, which is then 
far in the western horizon. 

Q. What are the principal stars in Taurus? 

A. The principal stars are two groups, the Pleiades and the 
Hyades, and the bright star Aldeharan. 



202 bouvier's familiar astronomy. 

Fig. 126. 




CONSTELLATION TAURUS. 
Q In what direction are the Pleiades from st Arietis ? 

A. Almost due east from a Arietis ; and in about an hour and 
a half after Arietis has culminated, the Pleiades will be on the 
meridian. 

Q. By what other name are the Pleiades known ? 

A, They are known by the name of the Seven Stars. 

Q. Where are the Hyades ? 

A. The Hyades are a group of stars, the brightest of which is 
about 10° degrees south-east of Alcyone, the brightest star of the 
Pleiades. 

Q. How can Aldebaran be found? 

A. Aldebaran, known also by the name of a Tauri, is the 
brightest star in the constellation Taurus, and is also reckoned as 
a star of the first magnitude or brilliancy. It may be found very 
readily, as it forms the letter V with two bright stars in the 
Hyades. 

Aldebaran is known as the " Bull'a Eye." 
Q. What OTHER bright stars are there in Taurus ? 

A. One in the north horn called /? or El Nath, and one in the 
point of the southern horn, about 10° to the south-east of ^. These 
two stars form the base of a triangle, of which Aldebaran is the 
apex. 

' GEMINI. 



Q. What is the next sign in order from Taurus ? 

A, Gemini, 



CONSTELLATIONS. 



203 



Q. How is Gemini designated on the globes and maps ? 

A. By the figures of two children sitting with their feet resting 
on the Milky Way. 

Gemini is the third sign, but the fourth constellation in the zodiac. 
Q. What are the principal stars in the constellation Gemini ? 

A. The principal stars are Castor and Pollux, 

Q. How can Castor be found in the heavens? 

A. It is nearly 30° east from M Rath, in the end of the Bull's 
northern horn. Castor is in the head of one of the twins. 

Q. Is there any thing remabkable about the star Castor ? 

A. Yes ; Castor, when viewed through the telescope, is a double 
star; the smaller one revolves round the larger in about ttuo hun- 
dred and fifty years. 

Q. To what CLASS of stars does Castor belong? 

A. To the second class; that is, it is a star of the second mag- 
nitude. 

Q. AVhere is the star Pollux situated ? 

A. It is about 4J° south-east of Castor, and a line drawn from 
the Pleiades through El Nath and extended, will touch it. 

Q. Is there any thing remarkable about Pollux? 

A. It is quadruple when seen through the telescope ; that is, 
it is composed of four stars. 

Q. By what names do astronomers denote these two stars, Castor and Pollux ? 

A. Castor is called a (Alpha) Greminorum, and Pollux /3 (Beta) 
Geminorum. 

The Twins are also known by the names of Castor and Pollux. 

Q. What OTHER STARS are there in the constella- 
tion Gemini? 

A. There is one called Wasat, or 3 (Del- 
ta) Geminorum, which is situated on the 
ecliptic about 8° south-west of Pollux. 
There is also a line of three conspicuous 
stars nearly parallel with Castor and Pol- 
lux, and about 20° south-west of them, 
which designate the feet of the Twins. 

Q. Are there any remarkable telescopic objects 
in this constellation ? 

A. There is a very singular double ne- 
bula situated a little south of the bright star 
Castor, which is partly surrounded by rings, 
with a bright star in the centre, of which ^(/. 
127 is a representation. 

Q. What OTHER singular telescopic object is there in 
the constellation Gemini ? 

A. About 2° south-east of AVasat is a iiebu- 
lous star, which is represented infy. 128, as 
seen through Lord Rosse's telescope. 



Fig. 127. 




Fig. 128. 




204 



bouvier's familiar astkonomy. 



CANCER. 
Q. "What is the fourth sign of the zodiac ? 

A. Cancer is the fourth sign, and ffth constellation, of the 
zodiac. 

Q. What REMARKABLE object is to be found in the constellation Cancer ? 

A. A nebulous group of minute stars visible to the naked eye, 
called Proesepe^ or the Beehive. It is situated about 15° south- 
east of Pollux. 

Q. What OTHER remarkable star is. there in Cancer ? 

A. Extend a line from Castor through Pollux, and it will reach 
a star called Tegmine, or ^ (Zeta) Cancri. The telescope shows 
it to consist of three stars, two of which are close together, and 
perform a revolution, one round the other, in about sixty years ; 
while the third one makes & grand revolution in about five or six 
hundred years. 

Q. What are the most conspicuous stars in Cancer ? 

A. There are no very bright stars in this constellation; the 
most conspicuous are a (Alpha) and ^ (Beta) in the southern claw, 
Asellus Borealis, north of Proesepe, and Asellus Australis on the 
ecliptic, about 3° to the south-east of Proesepe. 



LEO. 

Q. What is the fifth sigx of the zodiac ? 

A. Leo is the fifth sign, and sixth constellation. 

Q. What is the principal star in this constellation ? 

A. Regulus, or a Leonis, a star of the first magnitude, situated 
about half a degree north of the ecliptic ; it is also called the 
Lions Heart. 



Regulus culminates — that is, comes to the meridian- 
on the 20th of April. 



-about 8 o'clock in the eveniui 



Fig. 129. 




Q. Which is the next most brilliant 
star in this constellation ? 

A. It is called /9 or Dene- 
bola, a star of the second mag- 
nitude, in the Lion's tail, and 
about 25° east of Regulus. 

Q. What FIGURE is usually drawa 
on the Lion, in maps and globes ? 
A. The figure of a sickle. 

Q. What STARS form the sickle ? 

A. Those stars marked «, 
(Alpha,) which is Regulus, 5^, 
(Eta,) y, (Gamma,) ^, (Zeta,) 
and £, (Epsilon.) Regulus is 
at the end of the handle of the 



CONSTELLATIONS. 



205 



I 



sickle, and the other five stars form the ^ig- 130. 

blade. 

Q. Fur what is this portion of the constellation 

REMARKABLE > 

A. The November meteors always ap- 
pear to radiate from a point in the hend 
of the sickle, near the star y, (Gamma.) 

Q. Is the star y (Gamma) Leonis a remarkable 
one? 

A, It is. When viewed through the tele- 
scope it is a splendid double star, and moves 
in an orbit with a period of about a thou- 
sand years. 

Q. Does the telescope reveal any nebula in Leo ? 

A. Yes. In the lower jaw of Leo there 
is a beautiful spiral nebula, which is ren- 
dered very distinct by Lord Rdsse's large 
telescope. 

Fig. 129 is a correct representation of this singular ne- 
bula, which probably consists of many millions of star?. 
Q. Is there any other conspicuous object in this 

constellation ? 

A. Yes ; on the hind leg of Leo there 
is a very singular nebula, having some- 
thing of a spiral form in its centre. (See 
Fig. 130. 

VIRGO. 
Q. What is the sixth sign of the zodiac ? 

A. Virgo is the sixth sign, and seventh 
constellation, of the zodiac. 

Q. Has this constellation any conspicuous star? 
A. It has ; Spica, a star of the first mag- 
nitude, is represented in the Virgin's hand. 

Q. What is the position of Spica on the globe or 
map? 

A. It lies about 2° south of the ecliptic, 
and about 2^^ east o^ the autumnal equinox, 

Q. AVhat bright star is that in the Virgin's northern arm? 

A. It is called Vindemiatrix. It is a star of the third magni- 
tude, and comes to the meridian about three hours after Regulus, 
being situated almost due east from the last-named star. 

Q. What is the magnitude of the star in the southern shoulder? 
A. It is a star of the third magnitude, marked ^, (Beta,) about 
27° south-east from Regulus, both of which lie near the ecliptic. 

Q. Is there any remarkable star in this constellation? 

A. Yes ; a line drawn parallel to the ecliptic from ^, (Beta,) 




206 



BOUVIER S FAMILIAR ASTRONOMY. 



called also Zavijava, and extended 14°, would reach the star in 
the Virgin's side, called y (Gamma) Virginis. 

Q. For what is this star noted ? 

A. It is a binary star, — one revolving round the other in a 
highly elliptical orbit in the period of about one hundred and 
eighty years. 

Q. Mention the other prominent stars in this constellation. 

A. About half-way between Vindemiatrix and y (Gamma) is 
the star d, {Delta;) between j^ (Beta) and o (Delta) is a rich clus- 
ter of nebulce. About 10° south-east from o is ^, (Zeta,) on the 
knee : and about 12° east of Spica are two stars, 3° apart, which 
designate the Virgin's foot. 



•gi 



Fig. 131. 




The above figure represents a spiral nebula in the northern wing of Virgo, as it ap- 
pears through Lord Rosse's great telescope. 

LIBRA. 

Q. What is the seventh sign of the zodiac ? 

A, Libra is the seventh sign^ and eighth constellation. 

Q. When does the Sun enter this sign ? 

A. About the twenty-third of September ; but he does not 
enter the constellation till about the twenty-fifth of October. 
Q. How may Libra be distinguished ? 
A. It lies east of Virgo, and contains four stars — one of the third. 



CONSTELLATIONS. 207 

and three of the fourth, magnitude. These four stars form a tra- 
pezium. 

Q. How are the two divisions of the Scales distinguished ? 

A. The brightest star of the four, called Zubeneschamali, or a 
Libra, and the star south-east of it, forming the southern boundary 
of the square, are in the southern scale ; and the star marked /9, 
(Beta,) or Zuben-el-Gamabi, and the star marked y, (Gamma,) 
designate the northern scale. 

Q. Is there any remarkable object in this constellation ? 

A. About 7° south-east of Zubeneschamali is a nebulous clus- 
ter of minute stars, which Sir William Herschel pronounced to 
be a ''nebulosity not of a starry nature.'' This wonderful object 
is 12° due south of ^ (Beta) Libra. 

Q. Is there any other object worthy of note in this constellation ? 

A. About 12° due north of Zuben-el-Gamabi is a close cluster 
of stars, in which Sir William Herschel, with his large telescope, 
counted more than two hundred stars. 

Fig. 132. 




This splendid cluster, of which /?(/. 132 is a representation, was so closelv compressed 
in the centre that the individual stars could not be counted by Sir Wm. Herschel. 

SCORPIO. 
Q. What is the next sign in order from Libra? 

A. Scorpio^ the eighth sign, and ninth constellation. 

Q. In what month does the Sun enter this sign ? 

A. About the twenty-third of October; but he does not enter 
the constellation until about the twentieth of November. 

Q. What is the principal star in this constellation ? 

A. The brilliant red star of the first magnitude called Antares ; 
also called a Scorpii or Cor Scorpii 



208 



BOUVIER S FAMILIAR ASTROXOMY. 



Q. When can this beautiful star be seen on the meridian ? 

A. About 8 o'clock in the evening on the twenty-sixth of July, 

Q. Where is the next most conspicuous star in this consteUation ? 

A. In the Scorpion's head, about 8*^ north-west of Antares. 
This star is called /? (Beta) or Graffias. 

Q. How can the situation of Graffias be determined ? 

A. It is near the centre of an arc formed of several stars, which, 
together with Antares, resemble the body of a 2^aper kite, 

Fi?. 13.3. 




Firj. 133 represents the principal stars in the constellation Scorpio. The ri'ijhf hand, 
it will be remembered, is iceat, and the left hand, east, or contrary to terrestrial maps. 
Q. How are the other stars in this constellation designated ? 
A. South of Antares, stretching off to the south-east, there is 
a line of stars which form a curve resembling the tail of the kite. 

These stars form the tail of Scorpio. 
Q. Is there any remarkable object in this constellation ? 
A. About 4° north-west of Antares there is a globular cluster 
of stars in an apparent blank space in the heavens. The centre 
is one blaze of light, while the edges shade off 
into nebulosity. 

Whenever Sir William Herscbel found a blank space in the 
heavens, he was accustomed to say to his assistant in his obser- 
vatory — "Make ready to write, nebulae are just approaching." 
And such he always found to be the case. 

Q. What description of nebula may be found in Scorpio ? 

A. There is a cometary nebula in the tail of 

Scoi-pio, which is represented nifig. 134. 




CONSTELLATIONS. 



209 



SAGITTARIUS. 
Q. Which is the ninth sign of the zodiac? 

A. Sagittarius is the ninth sign, and tenth constellation, of the 
zodiac. 

Q. When is the Sun in this sign ? 

A. In November, and enters the constellation in December. 

Q. Are there any stars of the first magnitude in this constellation ? 

A. No ; there are no stars of the first or second magnitude in 
Sagittarius. 

Q. How may this constellation be found in the heavens ? 

A. It lies east of Scorpio, and can be distinguished by five 
stars, four of which form a quadrilateral figure ; the whole is 
called the Milk-dipper, as it lies partly in the Milky Way. 

Q. Describe the other parts of this constellation. 

A. Five small stars directly north of the Milk-dipper form the 
head of the Archer, and three others on the back of the animal 
form a small triangle. 

Q. Are there any conspicuous objects in this constellation? 

A. Not far from the point of the winter solstice is a fine globu- 
lar cluster of stars, which the telescope reveals as a rich group, 
with five minute stars near it. 

This cluster was for a time considered as a nebula, until Sir William Herschel first 
resulved it into stars by means of his great telescope. 

Fig. 135. 




Near this cluster is a beautiful nebula, having three lobes, forming together a circular 
iieioula with a dark space in the centre, in which may be seen three small bright stars 
by reference to the above figure. 

14 



210 



BOUVIER S FAMILIAR ASTRONOMY. 



CAPRICORNUS. 

Q. What is the next constellation east of Sagittarius ? 

A, Capricornus. 

Q. What SIGN and constellation is Capricornus in the order of the zodiac ? 

A. It is the tenth sign, and eleventh constellation. 

Q. When does the Sun enter the sign Capricornus ? 

A. On the 'list of December the Sun enters the sign, but does 
not reach the constellation till the 20^A of January. 

Q. What is that period designated by the entrance of the Sun into the sign 
Capricornus called? 

A. It is called the winter solstice. 

Q. Where is the Sun vertical at the time of the winter solstice ? 

A. He is vertical to all places on the Earth situated under the 
tropic of Capricorn. 

At this period the inhabitants of the northern hemisphere have their shortest days, 
and the north pole is in darkness. The inhabitants of the southern hemisphere enjoy 
summer, and the southern pole is illuminated. 

Q. Are there any brilliant stars in this constellation ? 

A. No ; none of the stars in this constellation are above the 
third magnitude. 

Q. Are there any remarkable telescopic objects in Capricornus ? 

A. There are some clusters, one of which, only the thirtieth 
part of a degree in diameter, contains hundreds of stars. 

Fig. 136. 




Fig. 136 represents a cluster situated about twenty 
degrees north-west of the bright star Fomalhaut. 
This cluster is very brilliant, and is supposed to be 
at a vast distance from our system. 



AQUARIUS. 

Q. What is the eleventh sign of the zodiac ? 

A, Aquarius is the eleventh sign, and twelfth constellation. 

Q. Where is this constellation situated ? 

A. To the east of Capricorn. 
Q. How^ is it represented on maps and globes ? 

A. By the figure of a man with an urn in his hand, from which 
runs a stream of water. 

Q. Are there any conspicuous stars in this constellation ? 

A. There are 7io stars of the first magnitude in the constella- 
tion Aquarius. 



CONSTELLATIONS. 



211 



Q, Mention the principal stars in Aquarius. 

A, The star a, (Alpha,) in the eastern shoulder, and /9, (Beta,) 
in the western, are of the third magnitude. Ancha, a star of the 
third magnitude, is in the waist of Aquarius. In the urn are 
five stars forming a Y, with the top of the letter pointing south- 
east. Scheat, a star of the third magnitude, is in the calf of the 
right leg. 

Q. Are there any remarkable objects in this constellation ? 

A, There is a planetary nebula in the constellation Aquarius, 
situated about eight degrees south-west of /9, (Beta,) in the 
shoulder. 

Q. What is this nebula remarkable for ? 

A. It is said to resemble the planet Venus, is very bright, and 
has a pale blue tint. Its disc is somewhat elliptical, 

Q. Has this object ever been measured ? 

A, It has. Sir John Herschel values its apparent diameter at 
20" angular measurement, which is the one hundred and eightieth 
part of a degree. 

Q, Has its magnitude ever been estimated ? 

A, It is supposed, if this object be only as distant from us as 
the stars, its real dimensions must be such as would fill the whole 
orbit of Uranus. 

Q. What would be the solid contents of a body large enough to fill the orbit 
of Uranus ? that is, a body whose diameter is three thousand six hundred mil- 
lions of miles ? 

A, Such a body would contain within its periphery more 
than sixty-eight thousand millions of globes as large as our 
Sun! 

Fig. 137. 



Fig. 137 is a representation of this nebula as it appears in a 
large telescope. 



Q. Is there any other telescopic object of interest in this constella- 
tion? 

A. About 10° north of the last-mentioned nebula is a fine 

globular cluster, which the telescope shows to be composed of 

tens of thousands of stars, 

Q. Is this object visible by the naked eye ? 

A. No; it requires the best telescopes to resolve it into stars, 
which are crowded together, as Sir John Herschel says, '' like a 
heap of fine sand." 




212 bouvier's familiar astronomy. 

Fig. 138. 




Fig. 138 is a representation of the above-named 
cluster. 



PISCES. 
Q. What is the twelfth sign of the zodiac ? 

A. Pisces is the twelfth sign, and/rs^ in order. 

Q. How is this constellation represented on the globes and maps ? 

A. It is represented bj two fishes some distance apart, united 
by a long ribbon, each end of which is tied round the tail of one 
of the fishes. 

Q. AYhat GREAT CIRCLE passes through this constellation ? 

A, The equinoctial colure. 

Q. "When the sun is in Pisces, what season is it ? 

A. The Sun enters the constellation Pisces and the sign Aries 
on the 20^A of March, at the time of the vernal equinox. 

Q. Are there any bright stars in the constellation Pisces ? 

A. No ; all the stars in this constellation are small, Ijii^g ^o- 
tween Aries on the east and Aquarius on the west. West and 
north-west of Arietis are eight or ten small stars, which form the 
Northern Fish ; the ribbon which attaches the fishes may be identi- 
fied by means of three or four small stars leading south-east as 
far as within a degree of the equinoctial, where we find the star 
El-Rischa, of the third magnitude, in the loop of the ribbon, 
which now takes a westerly course as far as the equinoctial 
colure, and is there united to the tail of the other fish. /3 (Beta) 
and y, (Gamma,) in the head, are the principal stars in the 
western fish. 

SECTION II. 

^[ortljern Constellations. 
Q. What is understood by the rising and setting of the stars ? 
A. Their appearance above and disappearance beneath the 
horizon. 

Q. Do all the stars sink below the horizon of all the places on the Earth ? 

A. No ; some stars never rise or set, but are always above the 
horizon, to the inhabitants north or south of the equator. 



CONSTELLATIONS. 213 

Q. How do those stars which never rise or set appear to move ? 

A, They appear to revolve round the pole of the heavens. 

Q. What are those stars called which never rise or set, but appear to move 
round the pole of the heavens ? 

A. They are called circumpolar stars. 

Q. What are the northern constellations ? 

A. Those constellations lying north of the zodiac, 

URSA MAJOR. 
Q. What is the most conspicuous of the northern constellations ? 

A. The constellation Ursa Major, or the Great Bear. 

Q. How can the constellation Ursa Major be identified ? 

A. By seven bright stars, which are on the meridian in the 
evening about the 5th of May ; and to all places between 50° and 
60° of north latitude they will be overhead. 

Fig. 139. 




"He who would scan the star,-\' skies. 
Its brightest geras to tell, 
Must first direct his mind's eye north, 
And learn the Bear's stars well." 

These seven stars are sometimes called the '' Wagon," and by some the '' Dipper j" 
four of the stars serve to form the bowl, and the other three the handle of the Dipper. 

Q. What are the names of the stars in the handle of the Dipper? 

A. Benetnasch or r^ (Eta) is in the end of the handle, or tail 
of the Bear, Mizar or ^ (Zeta) in the middle, and Alioth or e (Ep- 
silon) next the bowl of the Dipper. 

Q. What part of the constellation do these stars occupy ? 

A. They form the tail of the Great Bear. 



214 BOUVIER S FAMILIAR ASTRONOMY. 

When the constellation is on the meridian, between the zenith and the pole, these 
three stars in the tail point towards the east, or the left hand of the picture. The Ara- 
bians believed these seven stars to resemble a bier and mourners, hence the star in the 
end of the tail is called Benetnaach, which means in Arabic the chief, as it represented 
the chief of the mourners, 

Q. Is there any thing kejiakkable in either of the theee stabs in the tail 
of Ursa Major ? 

A. The middle star of the three, called Mizar, is double when 
seen through the telescope. 

By the naked eye a small star may be detected near Mizar, which, however, is more 
than 700" distant from it. This small star is called Alcor, and by some is thought to 
be the attendant of Mizar; but this mistake is rectified as soon as the telescope is ap- 
plied, which shows Mizar double, and Alcor at some distance from it. 

Q. What is the angular distance between Benetuasch and Mizar? 

A. Nearly seven degrees. 

Q. What is the angular distance between Mizar and Alioth, the third star in 
the tail? 

A. Ahout four 'degrees. 

Q. There are two stars in this constellation so situated that a line drawn 
through them and extended will pass through the north pole : what stars are 
they? 

A. When the Dipper is on the meridian, they are the two most 
tvesterly stars of the Dipper ; the name of the northern one is «, 
(xllpha,) or Duhhe; that of the southern is /3, (Beta,) or Merak. 
They are also called the Pointers, because they point to the pole. 

These seven stars constitute only apart of the constellation Ursa Major. 

"With, what a stately and raajestic step 
That glorious constellation of the north 
Treads its eternal circle !" 

Q. What are the names of the two other stars which, together with the Pointers, 
form the Dipper ? 

A. They are called d, (Delta,) or Megrez; and y, (Gamma,) or 
Phad. 

Q. What is there remarkable in the situation of Megrez ? 

A. It is situated within seven and a half minutes of the equi- 
noctial colure. 

Formerly the Grecians guided their ships at night by means of the constellation Ursa 
Major J and Manilius, a poet who flourished in the first century before Christ, says — 

" Seven equal stars adorn the Greater Bear, 
And teach the Grecian sailors how to steer." 

Q. To what places of the Earth are the seven stars of the Great Bear always 
above the horizon ? 

A. To all places north o^ forty-one degrees north latitude. 

Although the seven stars never set at all places at, and north of, 41° north latitude, 
yet the constellation is not wholly above the horizon at those places, for, by reference to 
the globe or map, it will be seen that the ichole of the constellation is always above the 
horizon at all places north of 68° north latitude. 

Q. How may the other parts of this constellation be known ? 

A. About fifteen degrees east of the Pointers is a collection of 



CONSTELLATIONS. 



21.5 



stars which form the face ; and when the constellation is on the 
meridian, a line drawn through the Pointers and extended about 
twenty degrees farther south, will reach two stars on the eastern 
hind foot ; about fifteen degrees west of these are two other stars, 
which form the other hind foot ; and about ten degrees farther 
west are two more stars similarly situated, which form one of the 
fore feet. 

Q. Are there any remarkable telescopic objects in the constellation Ursa 
Major ? 

A. About twelve degrees south of /9, (Beta,) one of the Pointers 
of the Great Bear, is a very singular nebula^ with two bright 
spots in the centre. 

Fig. 140. 



Fig. 140 is a correct copy of the nebula, as it appeared in 
the great telescope of Lord Rosse. 




Fig. 141. 



There is also a beautiful oval nebula about ten degrees 
south of Megrez, with a white nucleus, as will be seen by 
reference to fiq. 141. 




URSA MINOR. 
Q. In what constellation is the Pole Star ? 

A, In the constellation Ursa Minor. 

Q. How can it be designated ? 

A. A line drawn through the two Pointers, a (Alpha) and ,9, 
(Beta,) of Ursa Major, and extended nearly thirty degrees to the 
north, will designate the Polar Star. 



216 bouvier's familiar astronomy. 

Q. How MANY CONSPICUOUS stars are there in this constellation ? 

A. There are seven brighter than the rest, one of which is of 
the second magnitude, two of the third, and four of the fourth 
magnitude. 

Q. What FORM has this collection of stars? 

A. It is in the form of a dipper^ and bears a strong resem- 
blance to the seven stars in the Great Bear. 

Q. In what part of the constellation is the Polar Star? 

A. In the extremity of the handle of the Dipper ; it is repre- 
sented in the end of the tail of the Lesser Bear. 

Q. Does the Polar Star appear to move ? 

A. No ; unless viewed with a good instrument, it does "not seem 
to partake of the apparent diurnal motion of the other heavenly 
bodies, but appears perfectly stationary. 

Q. What EFFECT is produced in the appearance of the constellation Ursa Minor 
by the diurnal rotation of the Earth ? 

A. It appears as if swung round by the tail, the extremity of 
that appendage being at rest. 

Fi^. 142. 




In the above figure the constellation Ursa Minor is represented in four different posi- 
tions, as it appears to revolve round the pole. After every interval of six hours its 
situation changes from one position to the next, till the revolution is completed, the 
star in the tail, which represents the Polar Star, being apparently at rest. 

Q. Does the Pole Star occupy exactly the pole of the heavens? 

A. No ; it is one degree and a half from the pole of the 
heavens. It will gradually approach to within half a degree 



CONSTELLATIONS. 217 

from the pole, and in about 13,000 years hence it will be nearly 
49° from the celestial pole. {See Note 54.) 

In the infancy of navigation, before the compass was known, mariners were guided 
solely by the North Star, which Dryden thus describes : 

"Rude as their ships -were navigated then, 
No useful compass, or meridian known, 
Coasting, they kept the land within their ken, 

And knew no north, but when the Pole Star shone.' 

The Pole Star is also known by the names of Cynosura or Alrucaba, which last name 
is derived from the Arabic. 

Q. How does the Pole Star appear through a powerful telescope ? 
A. It appears double. 

Q. How may the other stars of the Little Dipper in Ursa Minor be found ? 

A. By extending a line through Megrez and Phad of the Great 
Dipper in Ursa Major, it will strike one of the four stars in the 
Little Dipper, called Kochab. The other three stars, together 
with Kochab, form the Little Dipper, of which the Polar Star 
occupies the end of the handle. 

About three and half degrees south-west of Kochab, when the latter is on the meri- 
dian above the pole, is a star marked y (Gamma) on the maps. This star and Kochab 
are called the Guards. 

Kochab is so called from an Arabic word Avhich signifies the North Star, because in 
the time of Ptolemy that star was nearer to the pole than the present Polar Star. 

LEO MINOR. 

Q. When Ursa Major is on the meridian, what other constellation is on the 
meridian south of it ? 

A. Leo Minor. 

Q. Where is Leo Minor situated ? 

A. It is situated directly between Ursa Major on the north, 
and Leo Major^ one of the zodiacal constellations, on the south. 
Q. Does Leo Minor contain any first-class stars? 

A. No ; Leo Minor contains no stars of the first or second mag- 
nitude, and but one of the third, the others being still less brilliant. 

Q. How may the principal star in Leo Minor be designated ? 
A. It forms nearly an equilateral triangle Avith Denebola and Re- 
gulus ; the former is in the tail, the latter in the heart, of Leo Major. 

Q. How may the other stars of this constellation be found ? 

A. They consist of a collection of small stars which lie chiefly 
to the north of the head of Leo Major. 

This constellation is on the meridian at the same time as Regulus. 
Q. Are there any remarkable telescopic objects in Leo Minor ? 

A. There is a bright nebula four degrees from ^ (Xi) Ursa Ma- 
jor, on a line between that star and Regulus. 

Q. What APPEARANCE has this object when seen through the telescope? 

A. It is a bright globular nebula, which more powerful instru- 
ments than those now in use may resolve into stars, showing it to 
be a grand globular cluster. 



218 



BOUVIER S FAMILIAR ASTRONOMY. 



''In other words, not only suns beyond suns, but glorious systems of suns arranged in 
harmonious order." Our solar system, although it is five thousand seven hundred viiU 
lions of miles in diameter, is a mere poi»< in the universe, Smyth says — "Should it 
(our system) be taken for an average among the millions of suns around, what imagina- 
tion can grasp the immensity of creation ! Indeed, where system thus stretches beyonJ 
system, the space must be infinite; and in such contemplation we become conscious of 
our own littleness. No subject whatever, except revelation, can give a more exalted 
conception of the Eternal Fountain of all intelligence." 



CANES VENATICr. 
Q. Where is Canes Venatici situated ? 

A. A little to the south-east of the Great Bear. 

Fi-. 143. 




CONSTELLATIONS. 



219 



Q. How is this constellation represented on globes and maps ? 

A. By two greyhounds tied together by a ribbon around their 
necks. 

Q. What are the names of these greyhounds ? 

A. The northern is called Aster ion^ and the southern one, Char a, 

Q. Is there any remarkable telescopic object in this constellation ? 

A, Yes ; a wonderful nebula exists near the ear of Asterion, 
the northern Hound, and not far from Benetnasch, in the tail of 
Ursa Major. 

Fig. 143, on the opposite page, represents this nebula as seen through Lord Rosse's 
telescope. (*S'ee Note 55.) 

Q. What OTHER remarkable telescopic object is there in Canes Venatici ? 

A. There is a great globular cluster just east of the southern 

Hound. 

Q. Does this cluster consist of many stars ? 

A. It consists of not less than one thousand small stars, having 
a blaze of light in the centre, and a sort of fringe of stars round 
one side. 

Fig. 144. 



The cluster represented in fig. 144 is one of those 
closely-compacted combinations of stars which 
indicate some general bond of union or attraction 
between their constituents. 




COR CAROLI. 
Q. Are there any stars of the first magnitude in the constellation Cor Caroli ? 

A, No ; the brightest star called Cor Caroli, or 12 Canum Ye- 
naticorum,''' situated in the new constellation Cor Caroli, is of the 
third magnitude. 

The constellation Cor Caroli is situated between the Greyhounds Asterion and Chara. 
This constellation was formed by Halley, and called Cor Caroli, or Charles's Heart, at 
the suggestion of Sir Charles Scarborough, who was a great mathematician, and also 
physician to King Charles. 

Q. How can Cor Caroli be detected on the map or in the heavens? 

A. Its place may be found by drawing an imaginary line from 



* Canum Venaticorum is the genitive of Canes Venatici. 



220 



bouvier's familiar astronomy. 



the Pole Star through Alioth, in the tail of the Great Bear, when 
that constellation is on the meridian, and extended seventeen de- 
grees farther south. 

Cor Caroli appears a double star when viewed through a good telescope. 
Q. Are there any remarkable objects in this constellation ? 
A. Except Cor Caroli, this constellation offers nothing remark- 
able to the unassisted eye. 



Fig. 145. 



COMA BERENICES. 




). How may the constellation Coma Berenices be dis- 



tinguished 



A. It lies south of Canes Venatici, and iiorth 
of the tail of Leo. 

Q. Has Coma Berenices any conspicuous stars ? 

A. It has not : all the stars in this constel- 
lation are belozv the third magnitude. The 
principal cluster lies between Cor Caroli and 
Denebola, in the tail of Leo. 

Q. Are there any wonderful telescopic objects in 
this constellation ? 

A. About twenty degrees west of the bright 
star Arcturus the telescope reveals the mag- 
nificent nebula represented in the annexed 
figure. 

BOOTES. 

Q. How is Bootes represented on the globes and maps ? 

A. He is represented as a man with a club in one hand ; the 
other is raised in which he holds the ribbon which is attached to 
the hounds Asterion and Chara. 

Bootes is called the Bear-driver, and is represented as driving the Great Bear round 
the north pole. 

Q. What is the principal star in the constellation BocJtes ? 

A. Arcturus, or a (Alpha) Bootes. 

Q. How may Arcturus be found ? 

A. About the middle of June, at nine o'clock in the evening, 
it may be seen on the meridian. An imaginary line drawn 
through the two stars in the end of the tail of Ursa Major, and 
extended about 30°, will point out Arcturus. 

Spica in Virgo, Denebola in the tail of Leo, Cor Caroli, and Arcturus form a quadri- 
latei-al figure. 

Q. There is a star in each shoulder of Bootes : how can they be found ? 
A. About tiventy degrees east of Cor Caroli is y (Gamma) or 
Seginus, in his left shoulder, and about ten degrees south-east of 



I 



CONSTELLATIONS. 221 

Seginus is S, (Delta,) in the right shoulder. These two stars 
form, with Arcturus, an isosceles triangle, Arcturus being the apex. 

Q. What OTHER STARS are there in this constellation ? 

A, In the face is /9, (Beta,) which forms a triangle with the 
stars on the shoulders ; and a line drawn from d (Delta) to Arc- 
turus leaves Mirach or e (Epsilon) on the east. 

Q. Is there any thing remarkable about Mirach ? 

A. It is a double star, one of which revolves about the other 
in a period of nine hundred and eighty years. 

MONS M^NALUS. 
Q. By WH03I was the constellation Mons Msenalus formed ? 

A. By Hevelius. It is situated under the feet of Bootes, and 
contains no conspicuous stars. 

QUADRANS MURALIS. 
Q. Are there any conspicuous stars in the constellation Quadrans Muralis? 

A. There are not. 

Q. Where is this constellation situated ? 

A. Immediately north of Bootes. 

CORONA BOREALIS. 
Q. Where is Corona Borealis, or the Northern Crown, situated ? 

A. East of Bootes. 

Q. What is the form of this constellation ? 

A. It is of a circular form; seven stars serve to make the 
figure of part of a circle. 

Q. What is the principal star in this constellation ? 

A. The middle one of the seven: it is called Alphecca, or a 
(Alpha) Coronse Borealis. 

Alphecca comes from the Arabic words al feJckah, the dervish's cup or platter, from 
the incomplete circle formed by these stars. 

Q. Is there any telescopic object of note in Corona Borealis? 

A. Yes ; about ten degrees to the north-east of Alphecca is a 
binary star, marked c (Sigma) on the maps. One star of this 
system performs its revolution around the other in about five hun- 
dred and sixty years. 

HERCULES. 

Q. How is the constellation Hercules represented on the maps and globes ? 

A. By a man in a kneeling posture, clothed with a lion's skin, 
with a club in one hand, and the three-headed dog Cerberus in 
the other. 

Q. Where is the principal star of this constellation situated ? 

A. In the face of Hercules. 



222 bouvier's familiar astronomy. 

Q. What is the name of this star ? 

A. It is called a (Alpha) Herculis, or more generally Ras 
AlgethL 

Ras Algethi is a word derived from the Arabic raa-al-jdthi, the kneeler's head, because 
it is in the head of Hercules, who is in a kneeling posture. 

Q. When may Ras Algethi be seen in the meridian ? 

A. About the thirtieth of July, at nine o'clock in the evening. 

Q. What other conspicuous stars are there in this constellation ? 

A. One on the western shoulder, called /9 (Beta) or Korne- 
foroSj situated about midway between Ras Algethi and Alphecca, 
in Corona Borealis ; about three degrees to the south-west of 
Korneforos is a star marked y, (Gamma.) 

On the body of Hercules are four stars from five to eight degrees apart, which form a 
quadrilateral figure. 

Q. What remarkable telescopic object is there in the constellation Hercules ? 

A. Eight degrees north of Alphecca, in Corona Borealis, just 
under the western heel of Hercules, is a telescopic cluster or ball 
of stars of extraordinary brilliancy. 

Fig. 146. 




Dr. Nichol says, that "no representation can give a just conception of this magnificent 
cluster. Perhaps no one ever saw it for the first time through a telescope without 
uttering a shout of wonder." 

Q. What is \ (Lambda) Herculis noted for ? 

A. This star, A (Lambda) Herculis, situated in the eastern 
arm of Hercules, is noted as being the star towards which the 
whole solar system is slowly journeying. 

This theory of the slow motion of the solar system towards X (Lambda) Herculis waa 
first promulgated by Sir William Herschel; a hypothesis which the researches of M. 
Argelander serve to strengthen. 



CONSTELLATIONS. 223 

OPHIUCHUS.* 

Q. How is the constellation Ophiuchus kepresented on the maps and globes ? 
A. It is represented as a man holding a serpent in his hands. 

Q. Where is this constellation situated ? 

A, South of Hercules, the equator dividing it nearly equally. 

Q. What is the principal star in this constellation ? 

A, Mas Alhague, a star of the second magnitude, in the head 
of Ophiuchus. 

The name is derived from the Arabic rds-al-hawwd, "the serpent-charmer's head." 
Q. How can the place of Ras Alhague be identified ? 

A. It is situated about five and a half degrees south-east of Has 
Algethi, the bright star in the constellation Hercules. 

Q. Are there any stars in this constellation of the first magnitude ? 

A. There are not. There is a star of the third magnitude on 
the eastern shoulder, marked /9, (Beta,) and one of the fourth 
magnitude on the western shoulder, marked x, (Kappa,) which, 
with Ras Alhague, form a triangle. 

Q. How far below the equator does this constellation extend ? 

A. It extends about twenty degrees below the equator, where 
there is a star marked />, (Rho,) which designates the southern 
foot. 

The other stars belonging to this constellation may be easily learned by reference to 
the globe or map. 

SERPENS. 
Q. How is the constellation Serpens delineated ? 

A. By a serpent which Ophiuchus holds in his hands, the head 
of which is south of Corona Borealis, and between Bootes and 
Hercules ; it is then entwined round the western arm of Ophiu- 
chus, under his western knee, and passes into his eastern hand, 
makes one loop, and e-nds in the Milky Way. 

Q. Are there any conspicuous stars in Serpens ? 

A. There are none of the first magnitude ; one of the second 
magnitude is to be found by drawing a line from Alphecca, in 
Corona Borealis, nearly south twenty degrees. 

Q. What is the name of that star ? 

A. It is called a (Alpha) Serpens, or TJ ndk-al-Hag ^ which 
means the serpent's neck. 

Q. How is the head of Serpens distinguished ? 

A. By a cluster of five stars south of Corona and west of 
Hercules. 



* This constellation is sometimes called Serjaentanus. 



224 bouvier's familiar astronomy. 

Q. Which are the principal stars in the head ? 

A. Of the jive stars of the head, there are tivo of equal magni- 
tude, which are about two degrees apart ; the most westerly of 
these two stars is called /3, (Beta,) the other, y, (Gamma,) 
Serpentis. 

DRACO. 

Q. Where is Draco situated ? 

A. It is represented by a serpent, whose head is immediately 
tiorth of the eastern foot of Hercules, and after making several 
loops, its tail passes between Ursa Major and Ursa Minor ; thus 
it is said to separate the two Bears. 

Q. How can the head of Draco be distinguished 

A. About midway between the pole star and Ras Alhague is 
a bright star called /9 (Beta) Draconis, or Rastabe7i; about four 
degrees a little to the south-east of Rastaben is a star called y 
(Grhmma) Draconis, or Etanin ; north of Etanin is ^ (Xi) Dra- 
conis, or G-rumium. Four degrees north of Rastaben is a small 
star, which, with Rastaben, Etanin, and Grumium, forms a quadri- 
lateral figure. To the west of this four-sided figure is a small star, 
{El Makis,) which forms the apex of a triangle, the base being 
Etanin and Grumium. These five stars form the letter Y, and 
designate the head. 

Q. Which of the stars in the head is most noted ? 

A. Etanin. 

Q. Why is Etanin a noted star ? 

A. Because, as it passes very near the zenith of Greenwich, it 
was selected as being a suitable star to observe, in order to ascer- 
tain the par«?/aa; of the Earth's orbit, and by that means to ob- 
tain some general idea of the distances of the fixed stars. 

Q. Was the parallax of Etanin, y (Gamma) Draconis, discovered then? 

A. No : Dr. Bradley failed to discover the parallax of the 
star, although his observations were made with the most rigorous 
exactness, on a base comprising the diameter of the Earth's orbit, 
that is, of one hundred and ninety millions of miles ; yet this im- 
mense distance was not appreciable by any instruments then 
in use. 

From these observations, Dr. Bradley estimated the distance of y (Gamma) Draconis 
to be at least /our hundred thousand times more distant than the Sun. 

Q. What important discovery did Bradley make in his endeavors to determine 
the parallax of Etanin ? 

A. He discovered the aberration of light. 

Q. Is there any other noted star in Draco ? 

A. Yes ; there is one which is on the meridian with Arcturus, 



CONSTELLATIONS. 



225 



and is situated between the tail of Ursa Major ^ig- i^r. 

and the head of Ursa Minor. 

Q. What is the name of this star ? 

A. It is called Thuban, or a (Alpha) Dra- 
conis. 

Q. For what is Thuban noted ? 
A. About four thousand six hundred years 
ago it was the pole star. [See Note 54.) 

Q. How FAR is it from the pole now ? 

A. Nearly twenty-five degrees. 

Q. When Thuban was the pole star, how did it appear 
during a diurnal revolution of the Earth ? 

A. A diurnal revolution of the Earth causes 
all the heavenly bodies to appear to revolve ; 
but as Thuban then occupied the place of the 
pole, it appeared stationary. 

Q. Is any remarkable nebula to be seen, bj the aid of 
a telescope, in the constellation Draco ? 

A. Under the body of the Dragon is a bright 
oval nebula, very much elongated, with a dark 
line in the centre, as in fig. 147. 

CERBERUS. 

Q. Where is Cerberus situated ? 

A. East of Hercules. 

Q. How is Cerberus represented ? 

A. As a three-headed dog, which Hercules holds in hia hand. 

Q. Are there any conspicuous stars in Cerberus ? 

A. There are not. 




LYRA. 

Q. How is Lyra represented on the maps t 

A. By a harp. 

Q. Are there any bright stars in the constellation Lyra ? 

A. There is one star of the first magnitude in this constel- 
lation. 

Q. What is the name of this star ? 
A. Wega, or a (Alpha) Lyree. 

Q. Where is Wega situated ? 

A. It is the first bright star to the north-east of Ras Alhague, 

and with the Pole Star and Arcturus, it forms a large triangh\ 

It also forms a trian^^le with two small stars about two de^rrces 

from it. 

Q. Near what point in the heavens will Woga be situated at some future 
time ? 

A. Wega will be situated within five degrees of the north poh'. 

15 



226 bouvier's familiar astronomy. 

Q. How LONG will it be before Wega will be our pole star ? 

A. Wega will be our pole star about ten thousand years 
hence. 

Q. How FAR is Wega north of the equator ? 

A. About thirty-eight and a half degrees. 

Wega passes within a quarter of a degree of the zenith of the Washington Ob- 
servatory. 

Q. Has the distance of Wega from our system been estimated ? 

A. Yes ; its distance is computed to be more than one hundred 
and forty-two billions of miles from our Sun, 

Q. What OTHER STARS are there in Lyra ? 

A. Five degrees to the south-east of Wega is a star of the 
fourth magnitude, marked d (Delta) on the maps and globes ; 
about five degrees to the south of o (Delta) is another star, /9, 
(Beta,) of the third magnitude ; two degrees to the south-east of 
/5 (Beta) is y, (Gamma,) oi thQ fourth magnitude. 

Q. Are there any remarkable objects in the constellation Lyra ? 
A. A degree and a half to the north-east of Wega is the star 
£, (Epsilon,) which the telescope shows to be multiple. 

Fig. 148. This star appears double with even a low power; but 

by applying superior telescopes, each component of the 
double star is again divided, thus forming two pairs of 
stars, as will be seen in the figure. Between these pairs 
will be found three small telescopic stars of the thirteenth 
magnitude. Of these two systems, the stars composing 
that on the north (which is at the bottom of the figure) 
we will call A B, and those on the south, or top of the 
figure, C D. It is supposed that B will revolve round 
A in about two thousand years, and C around D in one 
thousand years, and that the two binary systems will re- 
quire nearly a million of years to perform their revolu- 
tion round the central ones. But what is this duration, 
when compared to that astounding unit of time — the grand 
revolution of the whole creation ! 

" To Thee I bend the knee ; to Thee my thoughts 
Continual climb ; who, with a master hand 
Hast the great Avhole into perfection touched." — Thomson. 

Q. What TELESCOPIC object is that situated between /? (Beta) and y (Gamma) 
Lyrae ? 

A, It is an annular nebula or ring. 

Sir AVilliam Herschel estimated the distance of this nebula from our system to bo 
nine hundred and fifty times that of Sirius — at least forty-seven thousand billions of 
miles. 

Q. Is there any other wonderful telescopic object in the constellation 
I-yra ? 

A. Ahont five and a half degrees south-east of /9 (Beta) is a 
globular cluster which is estimated at one thousand three hundred 
billions of miles from our system. 




CONSTELLATIONS. 227 

Fijr. 149. 



Fig. 149 is a representation of this superb 
cluster, with a semicircle of small stars 
near it. 




TAURUS PONIATOWSKI. 

Q. Where is Taurus Poniatowski situated ? 

A, Hheface of the animal is near the point where the solstitial 
colure crosses the equinoctial. 

Q. Are there any remarkable stars in this constellation? 

A. There are not. A few small stars form the letter V, which 
are thought to resemble the Hyades and Aldebaran in the 
zodiacal constellation Taurus. 

SCUTUM SOBIESKI. 

Q. How may this constellation be found on the globes and maps ? 

A. It is represented as a shield with a cross in its centre, im- 
mediately south of Taurus Poniatowski. 

Q. Are there any bright stars in Scutum Sobiesld ? 

A. There are none. 

Q. Are there any fine telescopic objects in this constellation ? 

A. There is a singular nebula in Sobieski's Shield. It was 
discovered by Sir John Ilerschel, who describes it as containing 
two small stars of a gray color. 

Fig. 150. 



Fig. 150 is a representation of this remarkable object. 



Q. What other wonderful telescopic object does this constellation contain ? 
A. There is one nebula in this constellation, immediately below 




228 



BOUVIER S FAMILIAR ASTRONOMY. 



Fig. 151. 



the Shield, which is in the 
form of a horseshoe; but 
when viewed with high mag- 
nifying power, it presents a 
different appearance. 

Sir William Herschel estimated this 
nebula to be nine hundred times 
farther from us than Sirius. In some 
parts of its vicinity he observed five 
hundred and eighfy-en/ht stars in his 
telescope at one time; and he counted 
two hundred and fifty-eight thousand 
in a space 10° long and 2h° wide. 

The annexed figure is copied from 
Sir John Hersehel's observations at 
the Cape of Good Hope, as it ap- 
peared through his telescope. 

Such is the wondrous arrangement 
of the celestial bodies ! Suns upon 
suns are thickly scattered throughout 
illimitable space, all obeying certain 
laws which were established by the 
fiat of the Great Eternal, for the pre- 
servation of that stability and un- 
broken harmony which characterizes 
all his works. 

••Amightjraaze, but not without 
a plan." — Pqpk. 



ANTINOUS. 
Q. Where is Antinous situated ? 

A. East of Taurus Ponia- 
towski and Scutum Sobieski, 
on the equinoctial. 

Q. How is Antinous repre- 
sented ? 

A. By the figure of a youth 
with a how and arrows in his 
hand. 
Q. Are there any stars of the first magnitude in this constellation ? 

A. There are not; there are none above the third magnitude. 

By reference to the globe or map, this constellation may readily be found, though 
the stars of which it is composed are not at all conspicuous. 




AQUILA. 

Q. What constellation is that immediately north of Antinous ? 
A. Aquila. 

Q. Are there any bright stars in Aquila ? 

A. Yes; there is a row of three bright stars in this constella- 
tion ; the middle one of the three is of t\iQ first magnitude. 



CONSTELLATIONS. 



229 



Q. What is this star of. the first magnitude called ? 

A. It is called a (Alpha) Aquilae, or Altair. 

The Arabians called this star el-tdir, " the flying eagle ;" whence the name Altair. 
Q. How is the row of three stars, of which Altair is the middle one, situated ? 

A, In a line south-east from Wega, in Lyra. 

Q. What is the name of the northernmost of these three stars ? 

A. Tarazed, or y (Gamma) Aquilse. 

Shahin-tara-zed, " the star-striking falcon," was a name applied to the constellation 
by the Persians ; but now the latter portion of the name is given to the northern of the 
three bright stars, also known as y (Gamma) Aquilas. 

Q. What is the name of the southernmost star in the row ? 

A, Alshain, or /3 (Beta) Aquilae. 

Alshain is a corruption of the Persian word al-shdhin, " the falcon." 
Q. What TWO bright stars are in the tail of the Eagle ? 

A. They are called e (Epsilon) and C (Zeta) Aquilse ; ^ (Zeta) 
is also called Deneb-el- Okdh, 

Deneb means "the tail," and el-Okd.b, "the eagle;" therefore ^ (Zeta) is called 
Deneb-el-Okab, because it is situated in the tail of the Eagle. 

Q. How may these two stars be identified ? 

A, They are about eighteen degrees due south of /9 (Beta) and 
y (Gamma) Lyrse. 

SAGITTA. 
Q. Where is the constellation Sagitta to be found ? 

A. It consists of a few small stars situated about ten degrees 
north of Altair, and is represented by the figure of an arrow. 



VULPECULA ET ANSER. 

Q. How is this constellation represented ? 

A. By a fox^ with his head towards the west, and having a 

goose in his mouth. 

Q. Are there any bright stars in Fig. 152. 

this constellation ? 

A, There is one of the third 
magnitude in the head of the 
fox, eighteen degrees north of 
Altair; the other stars in this 
constellation are of the fifth and 
sixth magnitudes. 

Q. Is there any remarkable tele- 
scopic object in this constellation? 

A. About fourteen degrees 
north of Altair is an elliptical 
nebula, the brightest part of 
which somewhat resembles a 
dumb-bell in shape. 

The above figure represents this object, as seen through Sir John Herschel's telescope. 




230 bouvier's familiar astronomy. 

Fig. 153 is the appenrance it assumed when viewed through Lord Ros^e's greiit tele- 
scope. The bright central part is resolved into stars, and a^'fringy appearance is visible 
on one side. 

Fig. 153. 




CYGNUS. 
Q. What constellation is that which lies east of Lyra and north of Vulpecula ? 

A. Cygnus. 

Q. Are there any bright stars in this constellation ? 

A. It is commonly known hj five bright stars situated so as to 
form a cross. This cross lies in the Slilky Way, about twenty 
degrees east of Lyra. 

Q. What is the brightest star in the cross ? 

A. Arided, Deneb, or a (Alpha) Cygni, is of the second magni- 
tude, and is situated at the top of the cross, or in the tail of the 
Swan. 

Q. About ten degrees west of Arided is a bright star of the third magnitude : 
what STAR is this ? 

A. It is d (Delta) Cygni, and forms the ivestern extremity of 
the cross-piece. This star d (Delta) is found to be double when 
viewed through the telescope, one of the stars appearing of a 
bright yellow, the other of a brilliant sea-green, 

Q. How can the other stars in the cross be distinguished ? 

A. To the south-east of d (Delta) is a bright star, which, with 
Arided, forms a triangle. The name of this star is y (Gamma) 
Cygni, or Sad'r, Imagine a line drawn from d, (Delta,) at the 
western extremity of the cross-piece, through Sad'r, and extended 
as far again ; this line would touch the eastern extremity of the 
cross-piece, in the star X, (Lambda;) then draw a line from Arided 
at the top, through Sad'r, and extend it twice as far, and you 
touch the star in the foot of the cross, which is called Albireo^ or 
^ (Beta) Cygni. 



CONSTELLATIONS. 231 

Q How is this constellation represented on the globes and maps ? 

A. By a swan flying down the Milky Way, the foot of the 
cross being on the bird's bill, and the extremities of the cross- 
piece representing its wings. The other stars of Cygnus are 
readily traced by a map. 

Q. What REMARKABLE Star is there in Cygnus ? 

A. A small star marked 61 on the globes, and known as 61 
Cygni, situated about 8° south-east of Arided. 

Q. For what is this star remarkable ? 

A. It is supposed to be one of the nearest stars to our system, 
and was the first which was observed to have parallax. 

Q. Is 61 Cygni a double star? 

A. It is ; and one of them revolves round the other in a period 
of more than five hundred years. 

Q. Why is 61 Cygni considered as being one of the nearest fixed stars ? 

A. Because it appears to have an extremely rapid and uniform 
motion through space towards some determinate though unknown 
region. This apparently rapid motion may be, in part, owing to 
the real motion of our solar system in space in an opposite direc- 
tion ; though, according to Sir William Herschel, we are approach- 
ing that quarter of the heavens. 

Q. What telescopic object is there in the constellation Cygnus ? 

A. A very singular nebula on the tip of the western wing. 

Fi<?. 154. 



The figure represents this object, which Herschel thinks ap- 
pears to constitute the connecting link between planetary ne- 
bulae and nebulous stars. 



CEPHEUS. 

Q. IIow is the constellation Cepheus represented ? 

A. By the figure of a king, with a croivn on his head and a 
sceptre in his hand. One foot rests on the solstitial cohere; the 
other is within a degree or two of the Polar Star. 

Q. What is the principal star in this constellation ? 

A. Alderamin, a star of the third magnitude, in the western 
shoulder. 

Q. How may Alderamin be designated ? 

A. A line drawn from Polaris (the Pole Star) to u-irided, in 
Cygnus, will pass four degrees to the ivest of Alderamin, which 
lies a little more than midway between the pole and Arided. 




232 bouvier's familiar astronomy. 

Q. Describe the other stars in this constellation. 

A. About eiglit degrees north of Alderamin is /5, (Beta,) or 
Alphirk, in the side of Cepheus ; and fourteen degrees to the 
north-east of Alphirk is ^, (Gamma,) or Errai. In the head are 
three small stars forming a triangle^ one of which is variable, 

Q. Why is y, (Gamma,) or En-ai, worthy of note? 

A. Because in two thousand three hundred and sixty years 
hence it will be the Pole Star. 

Q. What remarkable telescopic object is there in Cepheus ? 

A. A brilliant cluster sixteen degrees due south of Errai, in 
the form of a triangle, with a small orange-colored star in its 
vertex. 

Ihis cluster is described as a mass of very small stars blended with nebulous matter. 
Q. When is Cepheus on the meridian ? 

A, About nine in the evening on the 10th of October. 

LACERTA. 

Q. What constellation, south of Cepheus, is on the meridian at the same time ? 

A. Lacerta. 

Q. Are there any conspicuous stars in Lacerta ? 

A. It is composed of a few small stars, but none are above the 
fifth magnitude. 

Q. Is there any remarkable telescopic object in this constellation ? 

A. About 30° west of Almach, in the foot of Andromeda, is a 
species of spiral nebula with a dark centre. 

Tiz. 155. 



The annexed drawing is taken from one made by Lord Rosse. 




HONORES FREDERICI. 

Q. Where is this constellation situated ? 

A. Immediately east of Lacerta. It contains but a few small 
stars. 

This constellation is not generally recognised by astronomers, it haviag, by some, 
been suppressed. 

DELPHINUS. 

Q. What constellation is that which comes to the meridian about the 15th 
of September ? 

A. Delphinus. 

This constellation is commonly known by the name of Job's Coffin. 



CONSTELLATIONS. 233 

Q. What are the principal stars in Delphinus ? 
- A. They are called a, (Alpha,) /9, (Beta,) y, (Gamma,) and o, 
(Delta,) and are situated so as to form a diamond. These stars 
are on the meridian at the same time as Arided in Cygnus. 
Immediately below these, to the south-west, is e, (Epsilon,) in the 
Dolphin's tail. 

EQUULEUS. 

Q. Where is the constellation Equuleus situated? 

A. South-east of Delphinus, near the head of Aquarius. 

Q. How is this constellation distinguished ? 

A. By a trapezium, sometimes called Kitalpha, consisting of 
four stars of the fourth magnitude, «, (Alpha,) /?, (Beta,) y, 
(Gamma,) and d, (Delta.) 

Kitalpha is from the Arabic kit-&t al-faras', " part of a horse." 

PEGASUS. 

Q. How is the constellation Pegasus delineated ? 

A. By the head and half the body of a horse, with wings attached. 

Q. What are the most conspicuous stars in Pegasus ? 

A, There are four bright stars, from twelve to seventeen de- 
grees apart, which form a quadrilateral figure, familiarly known 
as the Square of Pegasus. 

Q. Where are these stars situated ? 

A. Blarkah is in the shoulder of the Horse ; twelve degrees 
north of it is Scheat, on the fore leg ; about fifteen degrees east 
of Scheat is Alpheratz, which is situated on the equinoctial colure ; 
fifteen degrees south of Alpheratz is Algenih. These four stars 
form the Square of Pegasus. 

Q. There is a bright star in the Horse's mouth : what is its name ? 

A. It is called Enif. 

From the Arabic enf, " the nose." 
Q. Is there any remarkable telescopic object in Pegasus ? 

A. There is a globular cluster between the mouths of Pegasus 
and Equuleus, which Sir William Herschel estimated to be two 
himdred and forty -three times farther from us than Sirius. 

Fi<?. 156. 




The cluster in the figure is at an almost infinite distance beyond the small telescopic 
stars on each side of it. 



234 



BOUVIER S FAMILIAR ASTRONOMY. 



ANDROMEDA. 

Q. What is the name of that constellation east of Pegasus vrhich comes to the 
meridian about nine o'clock in the evening on the 20th of November ? 

A. Andromeda. 

Q. Ho-w is Andromeda bepresexted on the globes and maps ? 
A. Bj 3, female figure with chains on her wrists. 

Q. "What is the principal stab in the constellation Andromeda ? 

A. Alpheratz, or a (Alpha) Andromeda. 
Q. How may Alpheratz be fouxd ? 

A. It is situated fifteen degrees north of Algenib, on ,the equi- 
noctial colure, and forms the north-eastern star in the Square of 

Pegasus. 

Q. How may the other bright stars in this constellation 
be designated ? 

A. About fifteen degrees a little to the north- 
east of Alpheratz is the bright star /9 (Beta) 
Andromeda, or 3Iirach; and a line drawn from 
Alpheratz, through Mirach, ten degrees farther, 
will reach Almach, or y (Gamma) Andromeda. 

Q. Are there any remarkable telescopic objects in this 
constellation ? 

A. Besides the beautiful nebula represented 
in fig. 122, there is an elliptical nebula in the 
right foot of Andromeda, the centre of which 
appears hlacJc, and at each extremity of this 
black centre is a small star. 

Sir John Herschel notes this nebula in his catalogue aa 
*' a iconderful object." 

This nebula, discovered by Miss Caroline Herschel, is supposed to be of enormous 
dimensions. Its form is that of a flattened ring seen edgeways, and is placed at an in- 
conceivable distance from us. It consists, probably, of myriad>i of solar systems, which, 
taken together, are but a point in the universe. 

"Yet what is this, which to the astonished mhad 
Seems measureless, and which the baiflei thought 
Confounds? A span, a point, in those domains 
Which the keen eye can traverse." — Wars 




CASSIOPEIA. 

Q. What constellation is that situated due north of Andromeda ? 

A. Cassiopeia. 

Q. How is Cassiopeia bepbesented on the maps ? 

A. By di female seated in a chair, with her feet resting on the 
Arctic Circle. 

Q. How is the constellation Cassiopeia situated ? 

A. It is situated on the opposite side of the pole from the 
Great Bear. 



CONSTELLATIONS. 230 

Q. What FORM do the principal stars in this constellation assume, when taken 

COLLECTIVELY ? 

A, They are disposed so as to form the letter W. 

Q. How are these principal stars in Cassiopeia to be found ? 

A. About half-way between Alpheratz, in the head of Andro- 
meda, and the Pole Star, is Oaph. 

Q. What remarkable situation has the st;tr Caph ? 

A. It is situated on the equinoctial colure^ and is on the same 
side of the true pole as the Pole Star. 

As the Pole Star is not situated exactly at the Irue celestial pole, but withia 1'^ 32' 56", 
or about li°, of it, on the same side as Caph, the position of the Pole Star with regard 
to the pole may be known by observing the star Caph. 

Q. What is the next bright star east of Caph ? 

A. Shedir: it is situated about five degrees south-east of 
Caph. To the north-east of Shedir, and six degrees east of 
Caph, is Y (Gamma) Cassiopeise ; a little to the south-east of ;', 
(Gamma,) and nearly ten degrees east of Caph, is d (Delta) Cas- 
siopeise ; the fifth star, which forms the upper point of the W, is 
£, (Epsilon,) in the Lady's ankle. 

Shedir is so called from Al-aadr, "the breast," because it is situated on the breast of 
Cassiopeia. 

Q. Has any thing been discovered with respect to any of these five stars ? 

A. Yes ; Shedir is found to be variable^ its period from least 
to greatest brightness being about two hundred days ; y (Gamma) 
is also supposed to be periodically variable. 

Q. What KEMARKABLE OBJECT oncc appeared in this constellation? 

A. A neiv star suddenly burst forth in full splendor. 

Q. Where was the new star situated ? 

A. Near the eastern foot of Cassiopeia. 

Q. When did this star appear ? 

A. In November, 1572. {See Note 50.) 

Q. What appearance did this star assume during the time it was visible? 
A. At first it was white, then yellow, then red, and finally a 
blueish gray, 

Q. How LONG was it visible ? 

A» Sixteen months; that is, from November, 1572, to March, 
1574. 

Q. By what great astronomer was this star particularly observed ? 
A. By Tycho Brahe. 

Q. Are there any remarkable telescopic objects in Cassiopeia? 

A. About three degrees south-west of Caph is a magnificent 
cluster of stars, which was discovered bv Miss Caroline Herschel, 
in 1783. 



236 



bouvier's familiar astronomy. 

Fig. 158. 




This assemblage of stars, as represented in the figure, is only a small portion of a vast 
region of inexpressible splendor extending some distance around it. 

"Some sequestered star 
That rolls in its Creator's beams afar, 
Unseen by man ; till telescopic eye, 
Sounding the blue abysses of the sky. 
Draws forth its hidden beauty into light, 
And adds a jewel to the crown of night." 



TRIANGULUM. 

Q. Where is Triangulum situated ? 

A. It is a little to the south-east of Andromeda, and comes to 
the meridian with Almach. 

Q. How is this constellation REPRESENTED ? 
A. By a figure of a triangle. 

Triangulum is one of the old forty-eight constellations, and is supposed to h 
formed in imitation of the Greek capital letter A, {Delta.) Hevelius formerl : 
triangle, which he called Triangulum Minus; but it is no longer continued 
maps. 

Q. Are there any large stars in Triangulum ? 

A. No. The principal star is of the third magnitude, 

situated about seven degrees north-west of Arietis. 

Q. Are there any telescopic objects in Triangulum ? 

A. There is a remarkable nebula about ten degrees west 
principal star in Triangulum. 



ave been 

a !^ct•o;•.ll 
in tum« 



and is 
of the 



CONSTELLATIONS. 
Fig. 159. 



237 




This wonderful object is represented as it appears in Lord Rosse's great telescope. Ir 
was .supposed by Sir William Herschel to be of the three hixndred and forty-fourth order; 
that IS, three hundred and forty-four times the distance of Sirius from the Earth ; which 
would be the immense sum of nearly seventeen thousand billions of miles fromour planet. 

MUSCA. 

Q. Where is Muse a? 

A. It is a small new constellation, directly north of Aries. It 
contains no very bright stars ; the principal are three of the third 
and fourth magnitudes. 

This constellation was formed by Bartschius, the son-in-law of Kepler, for which 
reason Hevelius retained it in his catalogue. 



PERSEUS ET CAPUT MEDUSA. 

Q. Where is Perseus situated? and how is it represented? 

A. It is situated east of Andromeda and Cassiopeia, and north 
of Aries ; it is represented by the figure of a man with the head 
of Medusa in one hand, and a sivord in the other. 

The head of Medusa is covered with twining snakes instead of hair, and is sometimes 
noticed as a separate constellation, but astronomers usually unite it with Perseus. 

Q. When does Perseus come to the meridian ? 

A, It comes to the meridian at nine o'clock in the evening, 
about the tiventieth of December, 



238 BOUVIER*S FAMILIAR ASTRONOMY. 

Q. Where is the principal star in this constellation ? 
A. AhoMt fourteen degrees north-east of Almach is Algenih^ or 
a (Alpha) Persei, a star of the second magnitude, in Perseus's side. 

This star was formerly called Jfir/ak, 
Q. Where is there another bright star in this constellation ? 

A. In the head of 3Iedusa, It is called Algol, or ,9 (Beta) 
Persei. 

Q. Is there any thing remarkable about Algol. 

A. It is a variable star ; at its brightest it is of the second 
magnitude, after which it changes to a star of the fourth magni- 
tude. 

Q. What TIME does it occupy in these changes ? 

A. About two days and twenty hours. 

It varies from the second to the fourth magnitude in three hours and a half, and then 
increases in brightness up to the second magnitude in the same time; and for the re- 
mainder of the period remains at the brightness of the second magnitude. 

Q. Where is Algol situated ? 

A. About nine degrees south-west of Algenib. 

The other stars of this constellation can easily be found by consulting a map or globe. 
Q. What noted telescopic objects are there in this constellation? 
A. On the handle of Perseus's sivord is a magnificent cluster 
of stars, which emits a peculiarly splendid light when seen through 
the telescope. 

Fig. 160. 




The figure above represents this gorgeous group. Immediately following it is another 
splendid cluster of almost equal brilliancy. 

TARANDUS. 

Q. \Miere may Tarandus be found ? 

A. It is a small constellation represented by a reindeer^ directly 
north of Cassiopeia, and east of Cepheus. It contains no conspi- 
cuous stars. 




CONSTELLATIONS. 239 



AURIGA. 

<^. What constellation is that situated north of the Bull's horns and east of 
Perseus ? 

A. Auriga^ or the Chai'ioteer. 

Q. What is the chief star in Aui'iga ? 

A. Capella^ situated about 30° north-east of Aldebaran, and 
the same distance north-west of Castor in Gemini. 
Q. What star is that on the eastern shoulder of Auriga ? 

A. Menkaliiian, or /9 (Beta) Aurigae. It is situated about 
eight degrees east of Capella, and is of the second magnitude, 

Q. Are there any telescopic objects in Auriga worthy of note ? 

A. There is one situated in right ascension 5h. 20m. 51«., and 
declination 34° 6' north. 

Fig. 161. 



It will be seen by reference to the figure that in the centre of 
this nebula is a small triangle, consisting of three minute stars 
surrounded by a nebula. 



CAMELOPARDALUS. 

Q. What constellation is that situated north of Perseus and Auriga ? 
A. Camelopardalus. This constellation occupies that vast space 
between the Pole Star, Perseus, and Auriga. 

It was formed by Bartschius, and was only retained by Hevelius, when forming his 
new catalogue, on account of its having been introduced by Kepler's son-in-law. 

Q. Does Camelopardalus contain any bright stars? 

A. It contains no star above the fifth magnitude. 

LYNX. 

Q. Where is the constellation of the Lynx situated ? 

A. It lies between the Great Bear and Auriga, and north of 
G-emini and Cancer. 

Q. Does it contain any conspicuous stars ? 

A. It does not. 

This constellation is one of the new ones formed by Hevelius. 

TELESCOPIUM HERSCHELII. 

Q. Where is Telescopium Herschelii to be found ? 

A. It is a small constellation situated between the Lynx and 
Gemini. It contains no conspicuous stars. 

This constellation was formed by Bode, in honor of the discoveries of Sir William 
Herschel. 



240 bouyier's familiar astronomy. 

SECTION III. 

^outljeriT Constclhitious. 
Q. What are the southern constellations ? 

A. Those constellations situated south of the zodiac, 

MONOCEROS. 

Q. HoTv is Monoceros represented ? 

A. Bj a fabulous animal, somewhat resembling a horse, with 
one horn in the forehead. 
Q. How is Monoceros situated ? 
A. It lies east of Orion, the equinoctial passing through its centre. 

Q. Are there any stars of the first magnitude in this constellation ? 

A. No ; there are none above the fourth magnitude. 

ORION. 

Q. Where is the beautiful constellation Orion situated ? 

A. It is situated on the equinoctial, due south of the horns of 
Taurus, and west of the solstitial cohere. 

Q. When is the constellation Orion to be seen on the meridian in the eyeninp, ? 

A. About the twenty-fifth of JoMuary, at 9 o'clock in the even- 
ing, the centre of the constellation will be seen on the meridian. 

Fi-. 162. 




Fiij. 162 represents the constellation Orion. The right side of the picture is the west. 
and the left-hand side the east. The star marked a is Betelgeuse, on the east shoulder ; 
»n the west shoulder is Bellatrix, marked y. The three stars in his belt are ^, c, s; and 



CONSTELLATIONS. 241 

the one in his western foot, /?, is Rigel. A small triangle forms the face, the northern 
8tar of which is X. The semicircle on the west side of the picture are the stars which 
form the lion's skin. 

Q. Are there many bright stars in Orion ? 

A. There is one on each shoulder; three form his belt; and 
there is one in his foot. 

Q. How is Orion kepresented? 

A. By the figure of a man resting on one knee, with a lions 
skin in one hand and a club in the other, with which he is beat- 
ing Taurus. 

Q. What is the principal star in this constellation ? 

A. Betelgeuse, or a (Alpha) Orionis, a star of the first magni- 
tude, in Orion's eastern shoulder. 

Q. How may Betelgeuse be indentified ? 

A. By running an imaginary line through the stars in the end 
of the BulVs horns, and extending it about twelve degrees. 

Q. What is the name of the star in his western shoulder ? 

A. It is called y (Gamma) Orionis, or Bellatrix, and is nearly 
8° west of Betelgeuse. 

Q. What is the name of the bright star in Orion's western foot ? 

A, Rigel, or ^ (Beta) Orionis. It is a star of the first magni- 
tude, and with good telescopes appears double, 

Kigel derives its name from the Arabic Rijl-al-jauza, " the giant's leg." 
Q. What other well-known stars are there in this constellation? 

A. The three in his girdle, situated about midway between Be- 
telgeuse and Bigel. 

Q. What Greek letters are they known by ? 

A. They are known as d, (Belta,) s, (Upsilon,) and ^ (Zeta) 
Orionis ; which means that they are the d, s, and ^ of the con- 
stellation Orion. 

By seamen the three stars in Orion's belt were called the Golden Yard. They are 
also called Jacob's Staff, the Three Kings, the Hake, Our Lady's Wand, the Ell and 
Yard, kc A line drawn through the three stars in the belt, and extended to the north- 
west, will reach the Hyades and Pleiades. 

Q. There are three stars which of themselves form a small triangle, and which 
taken collectively form a triangle with Betelgeuse and Bellatrix, (the two stars 
on his shoulders:} in what part of the constellation are these three small stars 
situated. 

A. They arQ situated on hi^face. 

Q. What remarkable star is that situated just below the belt of Orion, about 
one degree south-west of ^, (Zeta.) 

A. It is c (Sigma) Orionis, a multiple star, which the telescope 
shows to be composed of ten members. 

Q. What remarkable telescopic object is there in the sword-handle of Orion? 

A. Near the star marked <p [Phi) is a wonderful nebula, con- 
sisting of hundreds of stars, which are surrounded by a great 
nebulosity which no telescopes have yet resolved into stars. 

16 



242 



BOUVIER S FAxMILIAR ASTRONOMY. 
Fix. 163. 




This nebula is known as the Fish's Mouth, to which it is thought to bear some resem- 
blance. Sir John Herschel has delineated it,-'- and given a catalogue of 150 of the stars 
contained in it, together with its nebulous branches and convolutions, which, owin'^; to 
the small size of our figure, cannot be fully shown. In large telescopes, to use Sir John 
Herschel's own words, it has the appearance of a "curling liquid; or a surface strewn 
over with flocks of wool ; or the breaking up of a mackerel sky." If, says the same 
author, the parallax of this nebula be no greater than that of the stars, its breadth can- 
not be less than one hundred times that of the Earth's orbit; that is, it must equal 
nineteen thousand millions of miles in diameter. But if, as is still more probable, this 
nebula be at a vast distance beyond the most distant stars, its magnitude must be incon- 
ceivably great. 



Orion's beams! Orion's beams' 

His star-gemm'd belt, and shining blade, 

His isles of light, his silvery streams. 

And gioom.y gulfs of mystic shade." — Manilitjs. 



CANIS MINOR. 
Q. How is the constellation Canis Minor represented ? 

A. It is represented by a small dog standing on, or just abov-e, 
the back of Monoceros, with its head towards the west. 

Q. Ave there any bright stars in Canis Minor? 

A. There are tivo : one of the first magnitude, called Procyon^ 

* See Observations at the Cape of Good Hope, p. 25, and Plale VIII. 



CONSTELLATIONS. 243 

is situated in the heart of Canis Minor ; the other, called Gomeisa, 
in his neck, is of the third magnitude. 

LEPUS. 

Q. What constellation is that immediately south of Orion ? 

A, LejpuSj or the Hare. 

Q. Are there any large stars in this constellation ? 

A. The largest is one of the third magnitude. There are four 
small stars in this constellation, which form a four-sided figure. 

SCEPTRUM BRANDENBURGIUM. 

Q. Where is the constellation Sceptrum Brandenburgium situated ? 
A, It is composed of a few scattered stars, west of LejJics. 

This constellation was formed in 1668, by Kirch, a German astronomer. 

CANIS MAJOR. 
Q. How is Canis Major kepresented ? 

A. By the figure of a dog resting on his hind feet, holding up 
his paws. His face is turned towards the west. 

The Tropic of Capricorn passes through the middle of this constellation. 
Q. When does Canis Major come to the meridian ? 

A. About an hour after Orion, or about 10 o'clock in the even- 
ing on the 25th of January. 

Q. What is the principal star in this constellation ? 

A. Sirius ; it is situated in the nose of Canis Major, and forms 
an equilateral triangle with Procyon in Canis Minor, and Betel- 
geuse in Orion's shoulder. 

Sirius is also known by the names of Canicula and the Bog Star. 
Q. Is Sirius a star of the first magnitude ? 
A. Yes ; it is the brightest star in the heavens. 

According to Sir John Herschel's experiments, the light of Sirius is found to be equal 
to three hundred and twenty-four times that of a star of the sixth magnitude, which is 
the smallest heavenly body visible to the naked eye. Dr. Wollaston estimated the light 
of Sirius to be less than the ticenty thousand millionth part of the Sun's light! But 
owing to the immense distance of this star from us, he concludes that its splendor, to an 
eye placed at 95,000,000 of miles from it, must be nearly equal to fourteen of our suns I ! 

COLUMBA. 
Q. Where is Columba situated ? 

A. South of Lepus and ivest of Canis Major and Argo. 

Q. How is it represented ? 

A, By a dove, with an olive-branch in its mouth. 

This constellation was introduced by Royer, in 1697. 
Q. Describe the stars in Columba? 

A. The principal star is a, (Alpha,) in the middle of the Dove's 
back. It is situated about ticenfy-tioo dcjrees south-west of Sirius, 



244 bouvier's familiar astronomy. 

and comes to the meridian one hour before it. ^ (Beta) and ) 
(Gamma) are two stars in the Dove's breast, south-east of a, (Alpha.) 
There are no remarkable stars in this constellation. 

officina typographia. 

Q. Where is the constellation Officina Typographia situated ? 

A. Directly east of Canis Major, and south of Monoceros. It 
is one of the new constellations formed by Bode. 

Q. Does it contain any stars of the first or second magnitude ? 

A. No ; it is composed of small stars, and contains some rich 
clusters not visible to the naked eye. 

CELA SCULPTORIA. 

Q. Where may Cela Sculptokia, or Praxiteles, be found ? 
A. It comes to the meridian about tiuo hours before Sirius ; and 
is on the meridian fifty-five degrees south of Aldebaran. 

It is one of the modern constellations, formed by Lacaille. 

EQUULEUS PICTORIUS. 

Q. Describe the constellation Equuleus Pictorius. 

A. It is one of the modern constellations, formed by Lacaille ; 
it is situated south of Columba, smd fifty -five degrees south decli- 
nation. The solstitial colure passes through it. It contains no 
conspicuous stars. 

ARGO NAVIS. 

Q. Where is the constellation Argo Navis situated ? and when does it come to 
the meridian ? 

A. It is situated south-east of 3Ionoceros and Officina, and east 
of Columba. The centre of this constellation comes to the meri- 
dian at 9 o'clock in the evening on the first of March. 

Q. Is Argo Nayis a large constellation? 

A. It is; and therefore astronomers divide it into four parts; 
namely, Argo Navis, or the Hull; Carina, or the Keel; Puppa, 
or the Stern ; and Velis, or the Sails. 

Q. Is this constellation visible in the latitude of Xew York ? 

A. Only the northern part of it is visible in that latitude ; but 
to those living under the Tropic of Cancer the whole constella- 
tion is above the horizon. 

Q. Are there any bright stars in Argo ? 

A. There is in this constellation a magnificent star of the first 
magnitude, called Canopies, which comes to the meridian fifteen 
minutes before Sirius. 

To an inhabitant living at 37° north latitude, Canopus will just graze the southern 
horizon at 9 o'clock in the evening on the 5th of February. Canopus is situated about 
53° «iouth of the equator. 



CONSTELLATIONS. 



245 



Q. Is Csinopus the brightest star in the firmament ? 

A. It is not as bright as Sirius, but it is a star of t\\Q first mag- 
nitude. 

Q. Is there any other star of the first magnitude in the constellation Argo ? 
A. Yes ; a star marked /?, (Beta,) about twenty-five degrees 
south-east of Canopus, is of the first magnitude. 

This star, called also Miaplacidus, is sometimes reckoned to be in the constellation 
Robur Caroli. 

Q. Name the other principal stars in Argo. 

A. About 24° north-west from /9 (Beta) is y^ (Gamma,) in the 
middle of the Ship. It forms a right-angled triangle with Cano- 
pus and ^. Seven degrees north-west of y is ^, (Zeta,) or Woas. 
This star is in a line with /9 and y. There are many other stars 
of the third, fourth, and fifth magnitudes in this constellation. 

Q. Is there any peculiarity about the star n, (Eta ?) 

A. Yes ; it is known to varg in brightness from the brilliancy 
of Sirius to a star of the fourth magnitude. 

The star 77 (Eta) is situated about 15° north-east of Miaplacidus. 
Q. Is there any remarkable telescopic object in this constellation ? 
A. The star r^ [Eta) is situated in a vast stratum of stars and 
nebulce, which renders it one of the most interesting objects of the 
southern hemisphere. 

Fig. 161. 




246 



BOUVIER S FAMILIAK ASTRONOMl, 



The great nebula surrounding rj Argus is found to consist of from 2000 to 6000 stars, 
wiiich have been revealed by the telescope, besides large nebulous tracts which defy 
the powers of the best glasses which have ever been manufactured. One of the areas 
which Sir John Herschel examined was estimated to contain more than^^yc thousand 
ttara in a square degree. 

Q. Is this nebula near the Milky Way ? 

A. It is seen through the Milky Way, but is computed to be 
at an in finite distance beyond it. 



Fig. 165. 




Sir John Herschel, in speaking of this ne- 
bula, says — " It is impossible for any one, with 
the least spark of astronomical enthusiasm 
about him, to pass soberly in review that por- 
tion of the southern sky ; such are the variety 
and interest of the objects he will encounter, 
and such the dazzling richness of the starry 
ground on which they are represented to his 
gaze." The same author has made a catalogue 
of 1216 of the principal stars in this wonderful 
nebula. 

Q. What OTHER telescopic object is to 
be found in Argo Navis ? 

A. Near rj (Eta) Argus there is 
a remarkable sidereal object like a 
comet, or curved wisp of light, with 
a double star in the head of it. 
Fig. 165 is a representation of it, 
as seen by Sir John Herschel. 



PYXIS NAUTICA. 



Q. Where is Pyxis Nautica situated in the heavens ? 

A. Immediately north of Argo. 

Q. Is this a large constellation ? 

A. No ; it is one of the new constellations formed by Lacaille, 
and contains no conspicuous stars. 



PISCES VOLANS. 

Q. How is this constellation represented ? 
A. By a fish, with long fins expanded. 



Q. Where is it situated ? 

A. Directly south of Argo Navis. 
in Pisces Volans. 



There are no bright stars 



ROBUR CAROLI. 

Q. Where is Robur Caroli situated? 

A. It is east of Argo, in the Milky Way. 



There is a bright 
star in Robur Caroli called Miaplacidus, though some astronomers 
consider it as belonging to Argo Navis, and distinguish it by the 
letter /9. 

This constellation was formed by Halley. 



CONSTELLATIONS. 247 



HYDRA. 



Q. How is Hydra represented on maps and globes ? 

A, As a huge snake, whose head is south of Cancer and points 
to the west ; its body then crosses the equinoctial, and, with va- 
rious circumlocutions, passes to the eastward, south of Leo, crosses 
the equinoctial colure south of Virgo and Libra, and its tail ter- 
minates at the claws of Scorpio, 

Q. Are there any bright stars in Hydra ? 

A. There is a star of the second magnitude in this constella- 
tion, called Cor Hydra, " the Serpent's Heart ;" this star is also 
called a (Alpha) Hydrse, or Alphard. 

This star makes a right angle with Procyon in Canjs Minor, and Regulus in Leo. 
There are other stars in this constellation, but none of them conspicuous. 

SEXTANS. 
Q. How is Sextans represented on globes and maps ? 

A, By the figure of a sextant, which is placed between Hydra 
and Leo. It contains no conspicuous stars. 

This constellation was formed by Hevelius, to commemorate the sextant which Tycho 
Brahe used at his observatory at Uraniburg, and which was subsequently destroyed by 
fii:e in his house in Dantzic, in September, 1679. 

FELIS. 

Q. Where is Felis to be found ? 

A. It lies on the meridian with Regulus, the principal star in 
Leo, and is south of Hydra. 

This constellation, represented by a cat crouching, was introduced by Bode ; it con- 
tains no conspicuous stars. 

ANTLIA PNEUMATICA. 
Q. How is Antlia Pneumatica represented ? 

A. It is represented by the figure of an air-pump. 

Q. Where is it SITUATED? 

A. Between Felis and Rohur Car oil. It contains no large stars. 

This constellation was introduced by Lacaille. 

CRATER. 
Q. Where is the constellation Crater situated? 

A. It is situated about the middle of Hydra, of which it is by 
some considered the same constellation. 

Q. Describe the principal stars in Crater. 

A. There are six stars of the fourth magnitude, which, taken 
together, form a beautiful crescent, which is open to the north- 
west. This crescent comes to the meridian about twenty-five 
minutes before Denebola, in the tail of Leo. It is one of the old 
forty-eight constellations. 



MS bouvier's familiar astronomy. 

COKVUS. 
Q. Where is Corvus situated ? 

A. South of Virgo and east of Crater. The equinoctial colure 
runs through this constellation. 

This is one of the old forty-eight constellations. 
Q. What is the principal star in Corvus ? 

A. Algorah, on the wing. It lies fifteen degrees south-west of 
Spica in Virgo : it makes an exact equilateral triangle with Spica 
and that noted star, y (Gamm^) Virginis. 

Algorab is derived from the Arabic Al-ghordh, " the raven." 

CENTAURUS. 

Q. How is Centaurus represented ? 

A. By a figure half man and half horse, the middle of the 
man being united to the shoulders of the horse. The man holds 
a spear in his right hand, with which he is piercing a loolf which 
he holds in his left. 

Q. Where is Centaurus situated ? 

A. The centre of Centaurus is to be found about fiftt/ degrees 
south of Virgo. Hydra and Corvus lie between Virgo and 
Centaurus. 

Q. What is the principal star in Centaurus ? 

A. A star known as a (Alpha) Centauri is one of the brightest 
and one of the most remarkable stars in the southern hemisphere. 

Q. Where is a (Alpha) Centauri situated ? 

A. In the Milky Way, sixty degrees south of the equator, and 
comes to the meridian a few minutes after Arcturus in the con- 
stellation Bootes. 

Q. For what is a (Alpha) Centauri remarkable ? 

A. It is remarkable as being the nearest to our system of any 
star with which we are acquainted. 

The star a (Alpha) Centauri is computed to be about twenty bi'llions of miles from the Earth. 
Q. Describe the other stars in Centaurus. 

A. In the horse's leg, five degrees west of a, (Alpha,) is /?, 
(Beta,) a star of the first magnitude ; about twenty-five degrees 
north of /9, (Beta,) is d, (Delta,) in his left shoulder; and eight 
degrees west of 3, (Delta,) is i, (Iota,) in his right shoulder ; these 
two stars, with //, (Mu,) in his breast, form a small triangle. Four 
small stars constitute his face. 

Q. Is there any remarkable telescopic object in this constellation ? 

A. About thirty-six degrees due south from Spica in Virgo is 
a small star called co (Omega) Centauri, which, to the naked eye, 
appears of the fifth magnitude, and a little hazy. When seen 
through a good telescope, it proves to be a globular cluster con- 
sisting of thousands of stars. 



CONSTELLATIONS. 
Fig. 166. 



249 




The above figure rex^resents this wonderful object, which, Sir John Herschel remarks, 
is "truly astonishing." There are other nebulae and some double stars in this constel- 
lation. There is a double star in the breast, which appears to be surrounded by a lumi- 
nous atmosphere. It is a very interesting object, but is only visible through the 
telescope. 

Fig. 167. 

Q. Is there any other remarkable object in Cen- 
taurus ? 

A. Near the fore feet of the Centaur is a 
remarkable nebula, which contains three small 
stars, one of which appears double, with a high 
magnifying power. Several minute stars may 
be detected in it. 

Fig. 168. 



Q. In what part of this constellation is the 
remarkable split nebula ? 

A. In the hreast of the Centaur is 
a remarkable nebula with two divi- 
sions, with a streak of nebulous light 
between them. 

The annexed figure is taken from a drawing by 
Sir John Herschel. 





260 bouvier's Familiar ASTnui-iOMy. 

AVIS SOLITARIUS. 
Q. Where is Avis Solitarius to bo found ? 

A. It is immediately south of Libra ; it is represented by an owl 
standing on Hydra, and its head within the limits of the zodiac. 

Q. Are there any conspicuous stars in this constellation ? 

A, No ; this constellation was formed by Le Monnier in the 
year 1776, and consists of small stars. 

LUPUS. 

Q. Where is the constellation Lupus ? 

A. It may be seen on the meridian on the 21st of June, at nine 
o'clock in the evening, south of Libra and Scorpio. 

Q. How is it REPRESENTED ? 

A. By a wolf, into which Centaur is about to plunge a spear. 
It contains no conspicuous stars. 

CIRCINUS. 
Q. What is that small constellation south of Lupus ? 

A. Circinus. It is situated in the Milky Way, and is com- 
posed only of small stars. 

CRUX. 

Q. There is a small constellation south of Centaurus, directly between the 
horse's feet : what is its name ? 

A, Crux, or the Cross. It is formed of four principal stars — one 
of the first magnitude, two of the second, and one of the third. 

Q. Where is the principal star situated ? 

A. In the lower extremity of the cross ; it is called a (Alpha) 
Crucis, and is situated about two degrees east of the equinoctial 
colure. 

Q. How are the other three principal stars situated ? 

A. One, six degrees north of a, (Alpha,) is called y, (Gamma ;) 
about /our degrees south-east of y (Gamma) is /3, (Beta;) and 
five degrees west of /9 (Beta) is d, (Delta.) 

Q. Are thei-e any telescopic objects worthy of note in this constellation ? 

A. About five degrees north of a (Alpha) is x (Kappa) Crucis ; 
near which is a beautiful cluster of stars of diff"erent colors and 
brilliancy. 

Q. Describe this cluster as it appears through the telescope. 

A. It occupies an area of only a forty-eighth part of a square 
degree, and yet it contains one hundred and ten stars, of the 
richest colors, giving it the eifect, as Sir John Herschel says, of a 
" rich piece of fancy jewelry." 

The central and principal star of this cluster is a deep red ; among the larger stars 
are some green, others blue, and all possessing great brilliancy. This beautiful con- 
gtellation is invisible to all the inhabitants of the Earth north of 35° north latitude. 



CONSTELLATIONS. 251 

MUSCA AUSTRALIS. 

Q. Where is Musca Australis situated ? 

A. Immediately south of Crux, and sixty-seven degrees south 
of the equinoctial. It is represented by the figure o^ afi// or beCy 
and contains no very conspicuous stars. 

This is a new constellation, introduced by Bayer. 

CHAMELEON. 

Q. Where is the constellation Chameleon ? 

A. South of Musca, and east of Pisces Volans. It consists of 
only small stars. 

This constellation is new, and one of the twelve introduced by Bayer, a German 
astronomer who lived in the seventeenth century. 

NORMA. 

Q. What CONSTELLATION is that situated east of Lupus, and between Circinus 
and Scorpio ? 

A. A new constellation called Worma, introduce 1 by Lacaille, 
a French astronomer who lived in the last century, and spent 
four years at the Cape of Good Hope, where he surveyed the 
whole southern hemisphere. 

In this constellation there is a singular cluster or knot of telescopic stars, which 
appears like a picture in the middle, and surrounded by a square frame, which is com- 
posed also of minute but bright stars. 

ARA. 

Q. What constellation is that east of Norma ? 

A. Ara, It is one of the old forty-eight constellations, and 
may be found south of the tail of Scorpio. 

Q. Describe the principal stars in Ara? 

A. Thirty degrees south-east of Antares in the heart of Scorpio, 
and twenty-two degrees north-east of a (Alpha) Centauri, is 7-, 
{Gamma ^) a star of the second magnitude ; about one degree 
north of y is /9, (Beta ;) six degrees north of /9 is a, a star of the 
third magnitude. The other stars in this constellation are of the 
fourth, fifth, &;c. magnitude. 

Ara is situated in the Milky Way. 

TELESCOPIUM. 

Q. AVhat constellation is that east of Ara and the tail of Scorpio ? 
A. Telescopium. 

This constellation was introduced by Lacaille. 

CORONA AUSTRALTS. 

Q. What is the name of the constellation situated between Telescopium and 
Sagittarius ? 



A. Corona Australis, one of the old forty-eight asterisms. It 
mtains no stars brighter than of the fifth magnitude. 

The telescope reveals some superb double stars in this constellation. 



252 bouvier's familiar astronomy. 

PAVO. 
Q. How is Pavo situated ? 

A. Pavo, one of the modern constellations, formed bj Hallej, 
is situated south of Telescopium, and south-east of Ara. 

Q. Describe the stars in Pavo. 

A. The principal, «, {Alpha,) a star of the second magnitude 
in the head of Pavo, comes to the meridian about 9 o'clock in the 
evening, on the 10th of September. About 10° south-east of a 
is ;', {Gamma,) a star of the third magnitude in the breast. This 
star, as well as /9, {Beta,) a star 4° to the west of it, are within 1° 
of the Antarctic Circle. A numerous assemblage of small stars 
constitutes the tail, which is expanded towards the west. 

TRIANGULUM AUSTRALIS. 

Q. Describe the situation of Triangulum. 

A. It is west of Pavo ; the Antarctic Circle passes through the 
middle of this constellation. 

Q. What are its prixcipal stars ? 

A. At its eastern angle is a, {Alpha,) a star of the second mag- 
nitude ; about six degrees north-west of a is /3, (Beta,) a star of the 
third magnitude, in the northern angle ; and eight degrees west of 
^ is y, (Gamma,) in the third angle of the triangle. 

The other stars in this constellation are small. 

APUS. 

Q. Where is Apus to be found ? 

A. It is east of Musca Australis, and within 13° of the south 
pole. Apus is a new constellation, introduced by Bayer. It is 
represented by a bird of paradise flying towards the east. All 
the stars in this constellation are very small. 

OCTANS. 

Q. What is the name of the constellation on which Pavo is represented as 
standing ? . 

A. Octans Hadleianus. This is a new constellation, introduced 
by Lacaille in honor of Hadley. 

Q. Does this constellation contain any bright stars ? 

A. There are no stars in this constellation above the fifth mag- 
nitude. 

The south pole of the heavens is in the constellation Octans. 
MICROSCOPIUM. 

Q. Where is the constellation Miroscopium situated ? 

A. South of Capricornus and east of Sagittarius. 

Q. Does it contain anj-- conspicuous stars ? 

A. It does not. It is one of the new constellations formed by 
Lacaille, and contains only small stars. 



CONSTELLATIONS. 253 

GLOBUS ^THEREUS. 
Q. What is that small new constellation east of Miroscopium and south of 
Capricornus ? 

A. G-lobus JEthereus, the Balloon. 

Q. Are there any large stars in this constellation ? 

A. No; there are none above Xhe fifth magnitude. 

PISCES AUSTRALIS. 
Q. Designate the place of this constellation. 

A, It lies immediately south of Aquarius, a:nd eight degrees 
south of the Tropic of Capricorn. 

Q. Does this constellation contain any large stars ? 

A. Yes ; there is one called Fomalhaut, of the^rs^ magnitude, 
in the eye of the Fish. 

Fomalhaut is derived from the ArvthXa fom-al-liut, "the Fish's mouth." It is still repre- 
sented in the mouth of the Fish on some charts, but is generally drawn to form the eye 
in new maps. 

Q. Describe the other stars in this constellation. 

A. About six degrees south-east of Fomalhaut is /5, (Beta,) a 
star of the third magnitude, in the fin ; and five degrees north-east 
of /9 is £, (Epsilon;) the three forming a triangle. 

The head of the Fish is towards the east, and its tail comes in contact with Globus 
iEthereus. 

Q. When does Fomalhaut come to the meridian ? and in what latitude is it 
VISIBLE ? 

A. It comes to the meridian on the twenty-fifth of October, at 
9 o'clock in the evening, and is visible to all places on the Earth 
except those situated north of the parallel of fifty-nine and a 
half degrees of north latitude. 

APPARATUS SCULPTORIS. 

Q. Where is Apparatus situated ? 

A. It is south-east of Pisces Australis ; the equinoctial colure 
passes through the middle of it. It contains no stars above the 
fifth magnitude. 

PHCENIX. 

Q. AVhat constellation is that situated south of Apparatus Sculptoris, on the 
equinoctial colure ? 

A. Phcenix. 

Q. Describe the stars in Phoenix. 

A. Twenty-two degrees south-east of Fomalhaut is a, (Alpha,) 
in the head ; it is a star of the second magnitude. Ten degrees 
east of a is ;', [Gamma,) in the eastern wing. 

TOUCANA. 

Q. There is a constellation immediately south of Phcenix through which the 
equinoctial colure and Antarctic Circle pass : what is the name of that asterism? 

A. Toucana. A star marked a, (Alpha,) of the third magni- 
tude, denotes the bill ; fifteen degrees south-east of this is /9, (Beta,) 



J 



254 bouvier's familiar astronomy. 

a star of the third magnitude, in the tip of the wing ; tj [Uta,) is 
situated about the middle of the body, and about twelve degrees 
south-east of a, (Alpha.) 

Q. Are there any telescopic objects in Toucana ? 

A. There is a star called 47 Toucani. 

Fig. 169. 




This magnificent cluster is copied from that in the " Observations at the Cape of Good 
Hope." Sir John Herschel calls it a "glorious object." It is one blaze of light in the 
centre, where may be seen a double star. 

GRUS. 

Q. Where is Grus situated ? 

A, South of Pisces Australis, and west of Apparatus Sculptoris. 

Q. Describe the stars in Grus. 

A, Eighteen degrees south-west of Foraalhaut is a, (Alpha,) in 
the back ; about five degrees south-east of a is /?, (Beta;) thirteen 
degrees north-west of /9 is y, (Gamma,) in the head. The other 
stars in this constellation are small. 

INDUS. 

Q. What constellation is that situated between Grus on the east and Pavo 
on the west ? 

A. Indus. 



CONSTELLATIONS. 255 

Q. How is it REPRESENTED On the maps and globes? 

A. By the figure of an Indian^ with a spear in one hand and 
a shield in the other. This is one of the new constellations intro- 
duced by Bayer. It consists only of minute stars. 

CETUS. 

Q. What is the name of that large constellation situated south of Aries and 
Pisces, and east of the equinoctial colure ? 

A. Cetus. It is the largest constellation of the firmament. 

Q. What star is that in the Whale's nose ? 

A. Menkar, or, as it is sometimes written, Menkab. It is 
situated ttventy-two degrees south-east of Arietis, and is a star of 
the second magnitude. 

It is called Menkar from the Arabic Al-minkhir, * the nose or snout." 
Q. When does Menkar come to the meridian ? 

A. At nine o'clock in the evening on the nineteenth of De- 
cember. 

Q. What very remarkable star is there in Cetus ? 

A. There is a star in the neck of the Whale which is variable. 
Sometimes it is a star of the second magnitude or brightness, 
when it diminishes in lustre until it becomes invisible ; after 
which it increases again in brilliancy, until it arrives to that of a 
star of the second magnitude. 

Q. What is the name of that remarkable star ? 

A. Mira. It is also known as o (Omicron) Ceti ; sometimes it 
is called i)!f2Va Ceti; it is also designated by some as Qollo Ceti., 
because it is in the neck of the Whale. 

Mira, from mirahilia, " wonderful." Thus Mira is called the " wonderful star." 
Q. What is the period of Mira ? that is, how long does it require to change 
from its greatest brilliancy to bright again ? 

A. Once in three hundred and thirty-one days, fifteen hours, 
and seven minutes. 

According to Cassini, a French astronomer, its period is 333 days ; Ismael Bullialdus 
estimated its time to be 334 days ; while Halley mentions that it passes through all its 
variations seven times in six years. Sir John Herschel calculated its period to be about 
twelve times in eleven years, or the more exact time of 331 days, 15 hours, 7 minutes. 
It remains at its greatest brightness about two weeks, decreases so as to become in- 
visible in about three months, remains invisible about five months, and continues in- 
creasing during the remainder of the period, or about 77 days. Hevelius, a celebrated 
German astronomer, who lived in the seventeenth century, states that he watched it 
constantly from 1648, and that from the years 1672 to 1676 it did not appear, although 
it was the particular object of his attention. On the 5th of October, 1839, it was un- 
usually bright. 

Q. Who DISCOVERED Mira? 

A. It was discovered in 1596 by David Fabricius, an astro- 
nomer, and disciple of Tycho Brahe. 

Q. About TEN degrees to the south west of Mira is a star of the third magni- 
tude : what is its name ? 



A. Baton Kaitos, or ^ (Zeta) Ceti. 



256 



BOUYIER S FAMILIAR ASTRONOMY. 



Fig. 170. 



Q. Describe the other principal stars in Cetus. 

A. About jive degrees west of Menkar, in the 
nose, is ;', (Gamma,) a star of the third magni- 
tude ; about five degrees south-west of 7' (Gamma) 
is ^, (Delta ;) thirteen degrees south of d (Delta) 
are four small stars, forming an irregular square ; 
they are £, (Epsilon,) ;:, (Pi,) c, (Sigma,) and p, 
(Rho.) This square lies about six or eight de- 
grees south-east of Baton Kaitos. About mid- 
way between Baton and Fomalhaut, in the 
Southern Fish, is /5, (Beta,) or Diphda, in the 
curl of the tail. In the extremity of the tail is 
i, (Iota,) Deneb Kaitos or Shemeli. 

Q. Is there any remarkable telescopic ob.ject in Cetus ? 

A. There is a long, narrow nebula in the tail 
of Cetus, about eight degrees south of ;9, (Beta.) 
It was discovered by Miss Herschel in 1783, and 
is described as having a singularly pale, milky 
tint. The annexed tigure is a representation 
of it. 

PSALTERIUM GEORGIAXUM. 
Q. How is the constellation Psalterium Georgian um re- 

PBESEXTED ? 

A. By a harp^ which is situated between th^ 
paws of the Whale and the feet of Taurus. 

Cetus, or the "Whale, is represented on the map as a monster Trith a tail like a fish, 
but hsLving paics somewhat resembling those of a dog. 

Q. Does tliis constellation contain any coxspicuors stars ? 

A. It does not. There is one star in this constellation, called 
Heid, which comes to the meridian with the Pleiades; it is a star 
of the fifth magnitude. 

This is one of the new constellations, and is not acknowledged by all astronomers. 
It was formed by Maximilian Hell, a Hungarian astronomer, in honor of George 11. 
of England. 




ERIDANUS. 
Q How is this constellation repbesexted on the maps and globes ? 

A. By a stream, which issues from between the paws of Cetus ; 
the principal branch runs eastwardly, then makes a bend towards 
the west and south-west, as far as Phoenix and Toucana ; this is 
called the Southern Stream. The other branch, or the Northern 
Stream, lies between Orion and the paws of Cetus. 

Q. Is there any star of the first magnitude in Eridanus ? 

A. Yes ; in the southern extremity of the stream is the beau- 
tiful bright star, Achernar, which is of the first magnitude or 



CONSTELLATIONS. 2')7 

brilliancy. It is situated fourteen degrees south of y (Gamma) 
Phoenicis, and forms a right angle with that star and a (Alpha) 
Toucani, in the bill of Toucana. 

Achernar is invisible to all the inhabitants of the Earth situated north of 32° nortt 
latitude. 

Q, Describe the other stars in this constellation. 

A. About eighteen degrees south-west of Rigel, in Orion's foot, 
is ;' (Gamma) Eridani, a star of the second magnitude ; it is some- 
times called Zaurak. 

The other stars in this constellation may easily be traced on the map. 



SCEPTRUM BRANDENBURGIUM. 

Q. Describe the constellation Sceptrum Brandenburgium. 

A, It is a small new constellation, situated between Lepus and 
Eridanus, and south of Orion and Taurus. It contains no very 
conspicuous stars. 

This constellation was formed in 1688 by Kirch, a celebrated German astronomer. 
His wife, Mary Margaret, was his assistant, and the author of several astronomical 
works. 

CELA SCULPTORIA. 

Q, Where is Cela Sculptoria situated ? 

A. It is directly south of Sceptrum Brandenburgium, and lies 
between Columba and Eridanus. It is represented by two graver's 
tools crossed, one over the other. It contains no bright stars. 



DORADO. 

Q. Where is the constellation Dorado, and how is it represented ? 

A. It is represented as a sword-fish, and is situated south of 
Equuleus Pictorius. The solstitial colure and the Antarctic Circle 
cross each other in the head of the Fish. This point is the pole 
of the ecliptic. 

Q. Where is the principal star of this constellation situated ? 

A. About fifteen degrees west of Canopus, in Argo Navis. 
The other stars in this constellation are small. 

Q. Are there any remarkable telescopic objects in Dorado ? 

A. About three degrees from the south pole of the ecliptic is 
a nebula, one of the most singular objects which the heavens pre- 
sent. It is situated in the thickest of the Nubecula, or Magellanic 
Clouds, and occupies about i\iQ five hundredth part of the Greater 
Nubecula. 

This was numbered 30 Doradns, by Bode, a German astronomer, who died in the early 
part of this century. 

17 



258 



BOUVIER S FAMILIAR ASTRONOMY. 
Fig. 171. 




This nebula contains numerous stars when seen through a good telescope. Sir John 
Ilerschel has made a catalogue of more than a hundred in this nebula alone. The re- 
presentation is taken from the drawing in his work, " Observations at the Cape of 
Grood Hope." 



Fig. 172. 



FORNAX CHEMICA. 

Q. Whei-e is Fornax Cliemica to be found on the map or globe ? 
A. It is situated in the hend of the Southern Stream of Eri- 
danus, to the west of the river. The principal star, called a, 
(Alpha,) lies about thirty-five degrees south- 
west of Rigel, in Orion's foot. The other 
stars of this constellation are not con- 
spicuous. 

Q. What TELESCOPIC object is there in this con- 
stellation ? 

A. There is a nebula, discovered by Sir 
John Herschel, which he describes as very 
bright in the centre, and being elliptical in 
its form. Three or four small stars appear 
near it. 

The annexed figure is copied from one by Sir John 
Herschel. 




CONSTELLATIONS. 259 



MACHINA ELECTRICA. 



Q. By WHOM was this constellation introduced ? 

A, By Lacaille. It is situated immediately south of Cetus, 
and north of Phoenix, and contains no bright stars. 



HOROLOGIUM. 

Q. What CONSTELLATION is that situated south-east of Phoenix ? 
A. Horologiiim. This constelhition was also introduced bj La- 
caille. It contains no very bright stars. 

SOLARIUM. 

Q. Where is Solarium situated ? 

A. East of Horologium, and, like it, contains no remarkable 
stars. 

■ RETICULUS RHOMBOIDALIS. 
Q. Describe the constellation Reticulus. 

A. It is represented by a large net, and is situated between 
Dorado and Horologium. 

This is a new constellation, formed by Lacaille. 

HYDRUS. 
Q. How is this constellation represented ? 

A, By a water-snake^ the head of which is between Toucana 
and Horologium. It winds round to the west, then to the east, 
touching Reticulus, and its tail ends at or near the south pole. 

The Nubecula Minor, or Lesser Magellanic Cloud, is in the constellation Hydrus. 
Q. Where is the principal star in Hydrus ? 

A. In the head^ about five degrees south-east of Achernar. 
There are two other stars of the third magnitude in Hydrus, d, 
(Delta,) in the middle of the body, and /9, (Beta,) in the tail, 
about ten degrees from the south pole. 

MONS MENS^. 

Q. What constellation is that south of Dorado, and between Hydrus and 
Pisces Volans ? 

A. Mons Mensce. It contains no large stars, but the Great 
Magellanic Cloud, or Nubecula Major, is situated in this con- 
stellation. 



PART ly. 

practical |,stronomg. 



"Here tTUths sutlirae, and sacred science charm, 
Creative arts new faculties supply ; 
Mechanic powers give more than giant's arm; 
And piercing optics more than eagle's eye." 

Practical Astronomy treats of astronomical instruments, and their applica- 
tion. These instruments are usually placed in a building called an observatory, 
which is erected in a suitable situation for obtaining an uninterrupted view of the 
heavens. 



CHAPTER I. 

(general froprties gI f igl]t. 

Before entering on a description of the astronomical instruments used in an 
observatory, it will be necessary to have some knowledge of the general pro- 
perties of light. 

Light is supposed to be caused by the undulations or vibrations of an elastic 
fluid called ether, which pervades all space. These undulations or vibrations, 
reaching the eye, affect the optic nerve, and produce the sensation which we call 
light. 

The progressive motion of light is generally believed to be undulatory ; that is, 
the rays of light traverse the space between the object and the eye in wavelike 
lines. Although light is supposed to be propagated from the luminous body in 
wave-movements through space, yet its rays are emitted \n parallel lines, and from 
all points in the luminous surface. 

Light requires time for its propagation, which has been fully proved by the 
discovery of the aberration of light, which has been already explained in a former 
part of this work. 

Light is found to be uniform in its motion ; rays from the Sun, a planet, or 
fixed star, move with the same velocity ; and as we know these bodies to be at 
very different distances from us, we conclude that the velocity of light is inde- 
pendent of the particular source from which it emanates, and the distance over 
which it has travelled before reaching our eye. The rate of motion of light 
through space is therefore considered as uniform, but its velocity is supposed to 
undergo a change when it enters such media as the atmospheres of the Earth and 
other planets. Its mean velocity is estimated at 192,000 miles per second. 
260 



GENERAL PROPERTIES OF LIGHT. 



261 



The illuminating power of any source of light is found to diminish as its dis- 
tance from the luminous body increases. A flat surface of a given area freely and 
perpendicularly exposed to a luminary at different distances, receives a quantity of 
light inversely as the squares of the distance from the luminary. 

Thus, a person situated at two feet from a candle has only one-fourth part of 
the light he would have if situated at one foot from it ; at the distance of three 
feet he would have nine times less ; and at four feet, sixteen times less light than 
if he was one foot from the candle. 

In order to ascertain the intensity of any given light, an instrument called a 
photometer is used, which, however, does not satisfactorily answer the purpose for 
which it was intended. 

Light is propagated in right lines, each of which is called a ray ; and each ray 
is supposed to issue from a luminous point in the illuminating body. 



Fig. 173. 




PENCIL OF LIGHT. 



A collection of parallel rays is called a beam of light ; 
and a collection of rays diverging from, or converging 
to a point, is called a pencil of light, as is represented 
in the annexed figure. 

That light is propagated in right lines, may be proved 
by the fact that we cannot see as well through a crooked 
tube as through a straight one ; and the outline of a 
shadow corresponds exactly with that of the object 
as seen from the luminous body. 

SECTION I. 
Refraction of figljt. 

Refraction is the inflection or bending of the rays of light out of their natural 
course. They always pursue straight lines, without any deviation, unless they 
pass obliquely from one medium to another, in which case they leave their origi- 
nal path and assume a new one. This change of direction of the rays of light is 
called refraction. After a ray has been refracted — that is, after it has taken a 
new direction — it then proceeds in a straight line till it meets with another me- 
dium, when it is again bent out of its course. 

A ray of light pursues a curved line when it passes obliquely through a medium 
the density of which changes uniformly. A ray of light from a heavenly body 
passes through our atmosphere in a curved line, because 
the different»strata of the atmosphere become uniformly 
more dense as it descends towards the Earth. But if 
it passes at once into a denser medium, its course is 
changed at the point of contact, into a path making a 
greater or less angle with the surface of the denser 
medium. 

Let A B be a ray of light, and B the point in which it 
enters from the air into the water. This ray, by the 
greater density of the water, instead of following its 
first direction, will be bent at the point B, and follow the 
line B C, which is called the refracted ray. 



Fig. m 


. 


A 


ID 


\ 


• 






--S=..=.==H^:=^! 


=^=i- 


, 


— V ■ 




\ 


: 


A-o- 



262 



BOUVIER S FAMILIAR ASTRONOMY. 



When light falls perpendicularly on the surface of a transparent medium, it 
passes through it without being refracted. In the above figure let the line D B 
repi'esent a ray of light ; its course is not enanged, but it pursues the line D B 
continued to E. Thus, a ray from a Star, or other heavenly body in the zenith, 
suffers no refraction, but reaches the eye of the observer in a line perpendicular 
to the plane of his horizon. 

A lens is a piece of glass, or other transparent substance, having its two sur- 
faces so formed that the rays of light, in passing through it, have their direction 
changed and made to converge and tend to a point beyond the lens, or to become 
parallel after converging or diverging ; or, lastly, to diverge as if they had pro- 
ceeded from a point before the lens. Lenses may be divided into convex and 

concave. 

Fig. 175. 





LENSES. 

Among convex lenses are the double convex, A, to which the appellation lens 
was originally applied, from its resemblance to a lentil seed (in Latin lens,) being 
bounded by two convex spherical surfaces, whose centres are on opposite sides of 
the lens. B represents a plano-convex lens, one side of which is bounded by a plane 
or flat surface, the other by a convex surface. C represents the meniscus lens, 
which is concave on one surface and convex on the other, the concave surface being 
part of the arc of a larger circle than the convex surface, the two surfaces must 
meet when produced. D is a double-concave lens, or concave on both sides. E re- 
presents the plano-concave lens, which is plane on one side and concave on the other. 
F is a convexo-concave lens, bounded by a convex surface on one side and a con- 
cave on the other ; but as the inner or concave surface is a portion of an arc of a 
smaller circle than that of the outer or convex surface, the two surfaces when pro- 
duced could not meet. 

The axis of a lens is a straight line drawn through the centre of its spherical 
surface. 

The images formed by lenses will be best understood by reference to the follow- 
ing figure : 

Fig. 176. 



A 


V I 


Li 


1 


^"^^^^xfixr^" 


\ 




^^<M><^C>\ 


I 


J 


l^^'^^ 



GENERAL PROPERTIES OF LIGHT. 



263 



A B is an object on one side of a double-convex lens, W V, but farther removed 
from it than the focal point F. The rays which proceed from A are united to a 
point at a, by passing through the middle of the lens, a being an image of A. In 
the same manner the rays from B pass through the centre and form an image at 6; 
thus, the object A B is represented inverted at a b. The size of the image is regu- 
lated by the distance of the external object from the lens. If the object be re- 
moved more than twice the distance of the focus, the image will be nearer, and we 
obtain a diminished image ; if the object be brought nearer to the lens, the image 
recedes, and is enlarged. In lenses of short focus the images lie nearer the glass 
than in those of greater focal distance. 

Fig. 177. 




Let A B(^5'. 177) be any object nearer the lens than its focal distance ; then the 
rays from it diverge on leaving the lens W V, and the eye placed at C, which is 
just at the focal point, sees an image at a b. The object and the image lie within 
the angle aob; but the object being nearest the glass, we see the image magni- 
fied. In microscopes, the images formed are of this kind. 

A ray of solar light is found to be composed of different colors, which may be 
separated from each other by means of a prism. If a beam of sunlight be ad- 
mitted into a dark room through a small aperture in a close shutter, and suffered 
to pass unobstructed, it would fall on a flat or plane surface — a sheet of paper, 
for instance — and form a circular disc of white light. 

Fig. 178. 




This disc is represented at E, and is formed by the ray D, which, before the 
prism ABC was placed in its path, followed the straight line D E. By placing 
tiie prism between the shutter and the screen M N, so that the ray may enter and 
quit it at equal angles, it will be refracted in such a manner as to form on the 



264 BorviER's familiar astbonomy. 

screen M N an oblong image, called the solar spectrum, wliich will be divided hori- 
zontally into seven colored spaces or bands of unequal extent, succeeding each 
other in the order represented — red, orange, yellow, green, blue, indigo, and violet. 
The red ray is the least, and the violet the most, refrangible of the spectrum. 



SECTION II. 

gcflcttion of figbt. 

The reflection of the rays of light is that property by which, after approaching 
the surfaces of bodies, they are thrown back or repelled. By the reflection of light 
we are able to discern all the objects around us, terrestrial as well as celestial. 

Rays of light which fall on rough or uneven surfaces are reflected in all direc- 
tions, and very irregulai-ly ; whereas, from smooth and polished surfaces, they are 
reflected with regularity. Highly polished surfaces are called mirrors or specula 
Mirrors may be classed into three kinds — plane, concave, and convex mirrors, ac- 
cording as they are bounded by plane, concave, or convex surfaces. The most 
familiar illustration of a plane mirror is the common looking-glass. 

When a ray of light falls upon a plane mirror, rather more than the half of it is 
reflected or thrown back in a direction similar to that in which it falls. Thus, if 
a ray fall perpedicularly on a plane mirror or looking-glass, it will be reflected per- 
pendicularly ; but if it approach the glass in an oblique direction, it will be re- 
flected in a line having the same obliquity as that by which it reached the glass. 
From these facts the following law has been deduced, namely : the angle of reflec- 
tion is, in all cases, exactly equal to the angle of incidence. This law holds good in 
regard to concave or convex mirrors, as well as to the plane mirror or common 
looking-glass. 

Let F C (flg. 179) be a plane mirror, and A B 
the line of incidence, or, as it is sometimes called, 
the incident ray ; then E B would be the line 
of reflection, for as the angle of the line of inci- 
dence is always equal to the angle of reflection, 
the angle A B D must be equal to the angle 
.Q D B E, which is the angle of reflection. 

In rolling a marble obliquely against a wall, 
the path of the marble from the hand to the wall would represent the line, or ray 
of incidence, the wall would take the place of the mirror, and the path of the mar- 
ble, as it would leave the point of the wall which it struck, would represent the 
line of reflection ; for the path of the marble towards the wall would have the same 
angle with the wall which its path on receding from the wall would have with it. 

Concave mirrors or speculas are of the greatest importance in constructing re- 
fracting telescopes. They cause parallel rays to converge, increase the con- 
vergence of converging rays, and diminish the divergence of diverging rays. 

In the following figure let A B represent a concave mirror, which, it will be 
understood, is the arc of a circle. The parallel rays fall upon it, and are reflected 
back, and converge at a point F, at a distance from the mirror equal to half the 
radius of which the mirror is an arc. The point of convergence F is called the 




GENERAL PROPERTIES OF LIGHT. 



265 



focal point, or focus of the mirror. And as the rays of light are always parallel 

to each other, if the mirror be exposed to the direct rays of the Sun, they would 

fall on the mirror A B, and be reflected back to the focal point F, where, if the 

quantity of rays collected by the mirror are ^uflScient, they would set fire to a 

combustible material. 

Fig. 180. 




Convex mirrors always cause the image to appear behind the reflecting surface. 

The image is always smaller than the object ; the image of a straight object always 

appears curved in a convex mirror, because the rays of light coming from the 

object are not all of the same length when they reach the mirror, on account of its 

convex surface. 

Fig. 18L 



<m 




-K 



In the above figure, A E B represents the convex surface of the mirror, and 
K A, J E, L B parallel rays falling upon it. These rays, when they strike the 
mirror, are made to diverge in the direction A G, B H, &c. and both the parallel 
and divergent rays are represented as they appear in a dark chamber, when a con- 
vex mirror is presented to the solar rays. 



H H H hi h 




tr5..= 

.Sis 
2 ^ « 




- B-m H M H E 



THE OBSERVATORY. 



267 



CHAPTER H. 

The best site for an observatory is a situation a little elevated above surround- 
ing objects, at a distance from manufactories or other buildings which emit much 
smoke ; it should also be at a distance from swampy ground, or valleys which are 
liable to be covered with fogs or exhalations. The ground should be gravel or 
other solid stratum, and not too close to a public road. It is generally considered 
important to have access to some distant field, where a pillar may be erected, on 
which to fix a meridian mark. This consists of some mark fixed on an immovable 
block of stone, situated exactly north of the observatory ; or in other words, a 
line drawn due north from the axis of a fixed telescope shall intersect the meridian 
mark. Sometimes there is also a meridian mark to the south of the observatory. 

Observatories are usually constructed with a revolving dome, and a door in it 
opening so as to admit a view from the zenith to the horizon. 

Fig. 183, 




268 ' BOUVIER*S FAMILIAR ASTRONOMY. 

The northern and southern horizon of the observatory should be unobstructed 
by trees or buildings ; and, if possible, the whole extent of the heavens should be 
visible. 

Fig. 183 represents a section of the rotary dome of an observatory. The two 
oblong doors a a are each nearly a foot "wide, opening outward. These doors 
meet at the apex of the dome, and when closed, the one which shuts last overlaps 
the other, thereby excluding the rain and snow. The circular broad rim h b, which 
in the figure appears like a straight line, forms the base of the dome ; c c is a 
similar ring, forming the wall-plate on which the dome rests and revolves. A cir- 
cular bed, formed partly by the dome and partly by the cylindrical framework, 
receives a set of small brass wheels, or cast-iron balls, of exactly equal diameters. 
On these the whole dome revolves, and may, by means of a crank, be turned to 
suit the wishes of the observer. The dome is usually furnished with an equatorial 
telescope. The above figure of a revolving dome is copied from Pearson's Prac- 
tical Astronomy, a work which stands pre-eminent in that branch of science. 



CHAPTER in. 

A TELESCOPE is an optical instrument, which serves for discovering and viewing 
distant objects. 

Telescopes may be divided into two classes — refracting and reflecting telescopes. 

SECTION I. 

Refracting ^dzscaps. 

The refracting telescope consists of lenses, through which the objects are seen 
by rays refracted through them to the eye. 

The lens or glass turned towards the object is called the object-glass, and that 
next the eye, the eye-glass. 

Refracting telescopes are usually fixed or mounted on a small brass pillar or 
tube, to which is attached three movable feet, which fold up when the telescope is 
not in use. 

Telescopes in observatories are usually firmly fixed on a pier of solid masonry, 
extending several feet below the surface of the ground. 

The aperture of a telescope is the opening of the tube or cylinder in which the 
object-glass is inserted. The rays proceeding from the object are united by the 
object-glass to form an image within the tube of the telescope. The distance be- 
tween the point where this image is formed and the object-glass, is called the 
focal length of the telescope. In order to magnify the image and render its de- 
tails more distinct, a microscope, composed of two or more lenses, is applied at 
the eye end : this is called an eye-piece, (one or more of which every telescope is 
provided with,) having different magnifying powers. A tube, worked by a milled 
head, connected with a rack and pinion, is made to slide in the main tube. This 



TELESCOPES. 269 

slidmg tube receives the eye-piece or microscope, the focus of which may be 
adjusted to suit the observer by means of the milled head. 

Eye-pieces are either positive or negative. The positive eye-piece consists of 
two plano-convex lenses, placed at a distance from each other less than the focal 
distance of the lens next the eye, with their plane side outwards, and their curved 
surfaces towards each other. This kind of eye-piece is used for telescopes with 
micrometers, and for transit instruments with spider lines in the focus of the 
object-glass. 

The negative eye-piece consists of two plano-convex lenses, having their curved 
faces in the same direction — that is, towards the object-glass — and placed at a dis- 
tance from each other nearly equal to half the sum of their focal lengths. 

There is another species sometimes used, called the diagonal eye-piece. This 
consists of a rectangular glass prism, or a flat piece of polished metal, inserted be- 
tween the two lenses of the eye-piece. The glass is preferable to the metal, 
because less light is lost or absorbed in the reflection. The piece of metal is 
placed at an angle of 45° to the axis of the tube; and as the ray of incidence is 
always equal to the angle of reflection, the ray is reflected into the axis of the 
eye-piece, which is situated perpendicular to the axis of the telescope, thereby 
admitting the eye of the observer to be applied to the side Lustead of the end of the 
telescope. 

From what has been said, the depression of the eye end of the telescope cannot 
incommode the observer, because his observations will be made at the side of the 
tube. But when very minute double stars are to be examined, the ordinary eye- 
piece is preferable, as less loss of light is sustained. 

SECTION II. 

In a darkened room, a ray of light received on a screen through a small aperture 
in a shutter, appears like a round white spot ; but when suff"ered to fall on a prism 
it is separated into a band of prismatic colors, the breadth of which is equal to the 
white spot, but considerably extended in length. This is called the dispersion of 
the rays. Different substances have different powers of dispersion ; for instance, 
two prisms of equal size, but constituted of different substances, will form spectra 
of different lengths. 

If a ray of white light fall at a certain angle on a prism of flint glass, and an- 
other ray fall on a prism of crown glass at the same angle, the spectrum formed 
by the flint glass will be found to be much greater than that produced by the crown 
glass. As the quantity of dispersion depends upon the refracting angle of the 
pi'ism, the angles of the two prisms may be made such that when the prisms are 
placed close together, with their edges opposed to each other, the one will coun- 
teract the action of the other, and will refract the colored rays equally, but in con- 
trary directions, so that an exact compensation will be effected, and the light will 
be refracted without color. 

Newton made many experiments on the passage of a ray of light through differ- 
ent media, but failed to discover that refraction may be produced without color. 



270 



BOUVIER S FAMILIAR ASTRONOMY. 



It was found that an object-glass composed of a single lens, instead of causing 
the rays to converge to one point, disperses them, forming a confused and colored 
image of the object ; but by constructing it of two lenses in contact with eacli other, 
and composed of diflFerent substances of certain forms and proportions, a perfectly 
Fig. 184. well-defined and colorless image is produced. The substances com- 
monly used for achromatic telescopes are one lens of flint and an- 
other of crown glass. 

The object-glass of an achromatic telescope consists of a convex lens 
AB [fig. 184) of crown glass, placed at the extremity of the tube to- 
wards the object, and another concavo-convex lens C D of flint glass, 
placed towards the eye. As the focal length of a lens is the distance of 
its centre from the point in which the rays converge, then the lenses 
A B and C D should be so constructed that their focal lengths shall be in 
the same proportion as their dispersive powers ; in which case they 
will refract rays of light without color. 

The principle of the achromatic telescope was discovered by Mr. Hall, 
of Warwickshire, England, in 1733; and in 1748, Mr. Dolloud, formerly a weaver 
in London, brought this discovery to perfection. 




SECTION III. 

Equatorial ^dcscope. 
The equatorial telescope has two axes of motion at right angles to each other 
When the instrument is properly adjusted, one axis is parallel to the axis of the 
earth, and is called the polar axis ; the other axis is parallel to the plane of the 
equator, and is called the declination axis. Each of these axes has a graduated 
circle attached to it, at right angles with its length. The motion required for an 
equatorially mounted telescope, is that parallel to the plane of the equator ; which, 
being uniform, is produced by means of an attachment of clock-work. This kind 
of instrument has one advantage over all others — namely, the object is retained in 
the centre of the field of view for hours, without any efi"ort on the part of the 
observer. 



SECTION IT. 

Cransit Instrument. 

The transit instrument is a telescope fixed at right angles to a horizontal axis, 
this axis being so supported that the line of collimation may move in the plane of 
the meridian. The axis, to the middle of which the telescope is fixed, should 
gradually taper towards its ends, forming cones well-turned and smooth, the bases 
of which are attached to the telescope. 

Transit instruments may be divided into two classes — portable and fixed. The 
portable transit, when placed exactly in the meridian and properly adjusted, may 
be used as a stationary iu'^trument in an observatory, provided the dimensions of 



TELESCOPES. 271 

the framework be such as to admit of a telescope of sufficient size. Telescopes 
having tubes more than 3^ feet focal length, are most advantageously placed on 
pillars of masonry, having a firm basis below the surface of the ground. 

The plate facing page 260 represents the transit instrument in the west wing 
of the National Observatory at Washington. In this room there is an opening 
20 inches wide, and extending entirely across the roof from north to south, and 
down the wall to within 4J feet of the floor on each side. 

The telescope T attached to this instrument is 7 feet in length, with an aperture 
of 5 inches. The instrument is mounted on two granite piers P P, 7 feet in height 
from the floor, and having a base of 2^ feet one way by nearly 2 feet the other 
way ; a a are two lamps for illumination, b h the supporting arms, and w w are 
compensations which preserve the lamps in an upright position; p p screens to 
soften the light ; e is a level attached near the object-end of the telescope. A is 
the handle of the screw which fastens the clamp end to the axis of the instrument; 
a slow motion in altitude is given to the telescope by means of the handles B B. 
S S are brass pillars placed on the top of the granite piers P P ; and on the top of 
each of the brass pillars is a lever, to the end of which is attached a weight W, which 
may be made to support part of the weight of the instrument by sliding it along the 
lever. At one end of each lever is attached a hook X, which supports two friction 
rollers under the ends of the axis; f ^ are the trunnions, and// the finding circles. 

Transit instruments are used to determine the time of the transit of a hea- 
venly body across the meridian, by which its right ascension can be accurately 
determined. 

SECTION V. 

Iieflecting Sbelcstope. 

Reflecting telescopes are those which represent the images of distant objects 
by reflection, chiefly from concave mirrors. 

By means of a concave mirror or speculum, an image of the object is formed, 
which is magnified by a lens. 

There are four kinds of reflecting telescopes — the Newtonian, Cassegrainian, 
Gregorian, and Herschelian, so called from their respective inventors. 

The Newtonian reflector consists of a tube open at one end, in which the 
rays of light enter parallel to each other and to the sides of the tube. Thesd 
rays fall on a concave mirror placed at the other extremity of the tube. 

Let A B E F {fig. 185) be the tube of the fj,,^ j85^ 

telescope, the end A F being turned so as to 

receive the light from the body to be viewed, ^ I^) A 

from which the parallel rays a b enter the .,, , 

tube and fall on the concave mirror B E, |^L---rTTrr-r7:rrr7r!7''! Y^ A 

which reflects them in converging lines to- K I-l jr 

wards their focus h. But between the focus 

and the large mirror B E, the plane or flat mirror G is fixed at an angle of 45"^, 
or at right angles to the axis of the tube. This small mirror is supported by an arm 
H, against which it rests, and being at right angles to the tube, receives the 
converging rays before they reach the focus h, and reflect them to I in the side of 



272 



BOUVIER S FAMILIAR ASTRONOMY. 



the tube where an eye-piece is placed, -which serves to view the image 
formed at the focus e. 

The Cassegrainian telescope consists of a cylindrical tube, the open end of which 
is directed towards the object. At the opposite end is inserted a concave spe- 
culum or mirror, with an aperture in the middle of it. Attached to the large tube 
is a small tube about the size of this aperture, at the extremity of which the eye 
is placed, thus enabling the observer to look through the hole in the mirror at 
a small convex mirror situated near the focus of the large mirror. At the end 
of the small tube the eye-piece is placed, which magnifies the image reflected 
into its focus by the small mirror. This kind of telescope is rarely used. 

The Gregorian reflector is similar to the Cassegrainian, except that the small 
speculum is concave instead of convex 

The kind of reflector most commonly used is the Herschelian. This consists 
of a tube, at the end of which is placed a concave mirror or speculum, a little 
inclined, so that the rays of light may be thrown a little to the side of the other 
end, which remains open, and is turned towards the object to be viewed. The 
observer sits with his back to the heavens, and looks into an eye-piece on that 
side of the tube towards which the rays from the large speculum throw the 
reflected image. The eye-piece magnifies the image thus reflected. 

Fig. 186. 




HERSCHEL'S FORTY-FEET REFLECTOR. 
The above figure represents the forty-feet telescope constructed by Sir William 
Herschel. The speculum, composed of metal and highly polished, was 48 inches 
in diameter, or just 4 feet, and weighed more than 2000 pounds. The tube was 
made of sheet iron; and the observer, mounted on the staging represented in the 
drawing, looked down the tube, which could be directed to any part of the heavens 



TELESCOPES. 



273 



by means of ropes and pulleys, and rollers on which the whole apparatus stands. 
This telescope is now not fit for use, having been injured on account of exposure 
to the weather. 

The Earl of Rosse has constructed a gigantic reflecting telescope, the tube of 
which is 6 feet in diameter, its focal length 54 feet, and has a speculum weighing 
four tons. Its superiority consists in the great quantity of light which it reflects, 
and the brilliancy with which it exhibits objects. It has a reflecting surface of 
4071 square inches, while Sir William Herschel's great telescope had only 1811 
square inches on the polished surface of its speculum. The cost of this instru- 
ment was about sixty thousand dollars. 

Fig. 187. 




The above figure represents the gigantic telescope of Lord Rosse, with its attach- 
ment to the heavy mason- work by means of chains and pulleys. The walls on each 
side are 24 feet apart, 72 feet long, 48 high on the outer side, and 56 on the inner 
side. The tube is 56 feet long and 1 inch thick, made of deal and hooped with iron. 

18 



I 



2T4 



BOUVIER S FAMILIAR ASTRONOMY. 



CHAPTER IT. 

The micrometer is an instrument attached to a telescope for the purpose of 
measuring distances in the field of view. It consists of movable threads or wires, 
situated exactly in the plane of the focus of the telescope, having a delicately gra- 
duated scale outside of the tube of the telescope, -whereby we are enabled accu- 
rately to measure the dimensions and relative angtdar distances of the images of 
the planets or stars. 

There are various kinds of micrometers, but that in most general use is the wire 
or spider-line micrometer, also called the filar micrometer. 



Fig. 188. 




11 



^^ 



The annexed figure, copied from the Rev. Dr. Pearson's great 
work on Practical Astronomy, represents Troughton's spider-line 
micrometer. This consists of an oblong brass box with a lid, 
which in the figure is represented as displaced, the interior ar- 
rangement being exposed to view. 

In the annexed drawing, the internal forks k and I are so nicely 
fitted into each other, and into the parallel sides of the oblong 
box, that when they are displaced by their respective screws o 
and/>, which are turned by the milled nuts m and n, there is not 
the least lateral shake ; and as the pins g and r, that pass into 
suitable holes in the metallic ends of the said forks, have each a 
spiral spring surrounding them, which press the forks back in a 
direction opposite to the action of their screws, there is no sensi- 
ble loss of motion whichever way the screws may respectively 
turn, which is an important condition in any construction where 
the measure depends on a screw. There are two spider's lines 
laid across the forks respectively, one attached to the prongs of 
^the inner fork k, and the other to the prongs of the outer fork I ; 
and the scratches in which the lines are embedded at the same time 
insure the parallelism of the tAvo lines, and prevent their touch- 
ing as they pass over one another, or lie in apparent contact. 
When these lines are well placed at the point of their apparent contact, they ap- 
pear as one line. The long line running at right angles to the small ones may be 
a wire, as its only use is to place the micrometer in a direction that shall take in 
both the objects viewed, between which the distance is required to be measui-ed, 
and to compare the position of that line with a true horizontal or vertical line, as 
the case may be ; and it is convenient to have this line well marked in the field 
of view. 

Notches answering to the size of the threads of the screw, which are divided into 
fives and tens, are made on one side of the field of view, thus — every tenth notch 
has a long notch, and between them every five has a rather shorter notch than 
every tenth, yet longer than the others in the scale. These notches are numbered 
from the point in the centre called zero, both ways towards the end of the box. 
The two spider lines may be made to pass one under the other, by means of the 




INSTRUMENTAL ADJUSTMENTS. 275 

screws, so that they -will coincide at the centre or zero. If the angular distance 
to be measured be considerable, as, for instance, the discs of the Sun or Moon, one 
of the lines may be drawn fifteen or twenty notches to the left of zero, and then the 
line moving to the right will complete the measure, when the sum of the two quan- 
tities will give the whole number of revolutions and parts indicated. When the 
angular distance to be measured is small, it will only be necessary to move one of 
the spider lines, the other being stationary at zero.* 

In order to render the spider lines in a Fig. 1S9. 

micrometer visible at night, they are illumi- 
nated by means of a small lamp attached 
to the side of the telescope ; thus, the eye 
of the observer is not exposed to the rays 
of the lamp, while the spider lines are ren- 
dered quite distinct. When the telescope 
is directed towards the south, the celestial 
objects enter the field of view from the west 
side, moving parallel to the horizontal wire. 
In the annexed figure, copied from that ex- 
cellent work. Smith's Cycle, the planet Ve- 
nus, in the form of a crescent, has just en- 
tered the field of view, and a star to the 
left is going off. 




CHAPTER y, 

SECTION I. 

A VERNIER is a scale or division adapted for the graduation of mathematical in- 
struments, so called from its inventor, Peter Vernier, a gentleman of Franche 
Comte, who communicated this discovery in the year 1631. 

The graduated arc of any instrument is called the limb. To divide or graduate 
the limb of an instrument, requires great accuracy and the most careful workman- 
ship. It is therefore desirable to make as few divisions as possible, as the smaller 
subdivisions are liable to be attended with inaccuracy. 

The principle on which a vernier is made, is to place a small arc, having the 
same radius as the instrument, so as to slide in coincidence with it; the small arc 
to be divided so that it shall contain, in the same length as the outer arc, one more 
division. Thus, if a certain space of the outer arc contain 19 equal divisions, the 
same space of the vernier shall contain 20 equal divisions ; therefore, it will be 
evident that each division on the vernier will be one-twentieth part less than a 

* See Kev. Dr. Pearson's Practical Astronomy, vol. ii. page 99. 



Z^b BOUVIER S FAMILIAR ASTRONOMY. 

division on tlie instniment, or that one degree of the instrument is equal to nineteen* 
twentieths of a division of the vernier. 

Fig. 190. 




J J-tJ 15 20 

The application of the principle will perhaps be more clearly understood on 
reference to the above figure, where each degree is seen divided into three parts, 
and where twenty divisions on the edge of the vernier or inner arc are commen- 
surate with nineteen on the limb or outer arc : the coincidence takes place at 15'' 
on the vernier with 25° on the limb ; and as the zero or stroke on the vernier 
points to three-fourths of the fii'st division of the twenty-first degree, the reading 
is 20° 15'' ; but if this first stroke had fallen in the second space of the same de- 
gree, the corresponding reading must have been increased by 20'', so that the 
angle measured would have been 20° 35''. 

SECTION II. 

In order to render the axis of a transit instrument perfectly horizontal, an 
instrument called a spirit-level is used. It consists of a glass tube ground so accu- 
rately within, that the upper side shall be, with regard to its length, very slightly 
convex upwards. This tube is nearly filled with alcohol or sulphuric ether, and 
hermetically sealed, so as to include a bubble of air. When this instrument is 
placed nearly level, the bubble of air contained within the tube will occupy the 
most elevated portion of the curve. 

The levels used for astronomical purposes may be divided into two classes — the 
riding and the hanging levels. 

The riding level consists of a tube nearly filled with alcohol or ether, and fast- 
ened to a flat piece of brass or bar of metal. When the level is to be used, it is 
placed on the part of the instrument intended to be rendered horizontal, and if the 
small air bubble stands midway between the ends of the tube, the instrument 
requires no adjusting; but if the bubble be more to the right than to the left of 
the centre of the tube, the right side of the instrument requires to be lowered, and 
vice versd, because the bubble always stands in the highest point of the tube. 
There is a graduated scale attached to the level, so accurately divided as sometimes 
to indicate even a second of angular deviation from a true horizontal position. 

The hanging level is sometimes attached to two arms, which are suspended from 
each end of the axis of the transit instrument. The situation of the bubble indi- 
cates the horizontality of the instrument, as described above, only the arms by 
which it is suspended may be unequal ; this may be tested by reversing it, and 
placing what was the right-hand arm on the left. If the bubble still stands in the 
same position with regard to the observer after the change has been made, the 
arms are of equal lengths, and the fault lies in the axis of the telescope. 



INSTRUMENTAL ADJUSTMENTS. 277 

SECTION III. 

The plumb-line, though now but little used, was formerly employed for the 
same purpose as the spirit-level — namely, for the horizontal and perpendicular 
adjustment of instruments. 

The plumb-line takes its name from the leaden weight which was formerly ap- 
pended to a fine thread of silk, and suspended freely from a fixed pin. As the 
direction of the string would naturally incline to the centre of the earth, its posi- 
tion was necessarily a vertical one. 

A fine thread of silver wire has since been substituted for the silken thread, as 
being less liable to vibrate ; and at the extremity of the wire a small brass bucket, 
nearly filled with shot, is appended, in the sides of which are some small holes. 
This bucket is suffered to hang in water to overcome any vibrations. 

The use of the plumb-line, though now nearly obsolete, was probably coeval 
with the construction of the first graduated instruments, to which it has been since 
used as an index. 

By means of the plumb-line applied to Dr. Bradley's zenith sector, the discovery 
of both the aberration of light and of the nutation of the Earth's axis were made, 
which data now supply corrections for reducing the apparent to the mean places 
of the heavenly bodies. 

The position of the plumb-line was referred to a very fine point made on the 
surface of a divided arc at zero, when the point of suspension was on or above the 
centre of the graduated limb. The permanency of the adjustments made by means 
of the plumb-line were not very reliable ; but these inconveniences were at length 
removed by the ingenuity of Ramsden, who substituted the image of a point for 
the point itself, which was made to fall exactly at the place of the plumb-line. 
This was effected by means of an optical contrivance grounded on the principle of 
the compound mici'oscope, which contrivance has been called Bamsden's ffhost. 

SECTION IV. 

grtificinl f ori^oit. 
That great circle of the celestial sphere which divides the upper from the lower 
hemisphere, called the horizon, is of no use to the practical astronomer, owing to 
the various inequalities of the Earth's surface. It is only in nautical observations 
made by reflecting instruments, where an allowance is made for the curvature of 
the surface of the ocean, that the natural horizon can be available. A reflecting 
plane, situated at right angles to a perpendicular line, may be substituted for the 
natural horizon, and is therefore called an artificial horizon. It is a well-known 
optical fact, that when a ray of light comes from a luminous body and falls ob- 
liquely on a reflecting plane surface, the angle of incidence on that plane, as it has 
reference either to the plane itself or to a line perpendicular to it, is always equal 
to the angle caused by reflection ; consequently, when a heavenly body is seen 
reflected from a perfectly horizontal plane, its apparent situation is just as much 
below the rational horizon as its real situation, when viewed by dii-ect vision, is 



278 bouvier's familiar astronomy. 

above it. Hence, all the heavenly bodies seen by reflection, give double the appa- 
rent altitude. 

Various fluids have been proposed as substitutes for artificial surfaces, of which 
water, oil, and mercury, well purified, have been found the most useful on trial. 
These fluids are generally contained in a shallow dish or vessel, which is some- 
times covered with a roof to pitch both ways, like the roof of a house ; the sides 
or top of the roof are of glass, to protect the surface from agitation by the wind. 
The incident ray passes through one glass or side of the roof, and after being 
reflected from the surface of the liquid, it passes out of the other side of the roof. 



SECTION V. 

%\z dLoIlimator. 

This instrument, invented by Captain Kater, enables the observer to determine 
at pleasure the place of the horizontal or zenith point on a vertical circle, without 
the assistance of the plumb-line, the spirit-level, or any reflecting surface, and 
that, too, at any time or moment of time. 

The principle of the collimator was first employed in 1785, by Da^dd Ritten- 
house, a celebrated American astronomer and mathematician. He made use of it 
for the purpose of fixing a definite direction in space, by the emergence of parallel 
rays from a material object placed in the focus of a fixed lens. There are two 
kinds of collimators, the horizontal and vertical. 

The horizontal collimator is a telescope of small dimensions, firmly attached to 
a cast-iron plate floating on mercury, and having a cross-wire in its focus. A 
telescope thus arranged, when placed on the surface of a basin of mercury, will 
always assume a horizontal position. By illuminating the cross-wires by means 
of a lamp, the rays from them will issue parallel, and may therefore be brought to 
a focus by the object-glass of any other telescope, in which they will form the 
image of any celestial object in their direction. Thus, the image may be viewed 
as an infinitely distant star, by an instrument attached to any mural or vertical 
circle. Since the axis of a floating telescope always preserves the same inclina- 
tion to the horizon, a reversed observation on opposite sides of the fixed circle, 
fixes the zenith point of that circle. 

The vertical collimator consists of a vessel of mercury, towards which the object- 
glass of a telescope, attached to a circle or a transit instrument, may be directed, 
so that the cross-wires in its focus may be reflected in the mercury. The wires 
of the instrument are illuminated by a lamp placed laterally, so that the rays 
from the lamp may be conducted to the wires without entering the eye of the 
observer; the telescope is then directed to the surface of mercury. The rays issue 
from the wires in parallel lines, and are reflected back to the object-glass, which 
is enabled to collect them again in its focus. By this means a reflected image of 
the cross-wires is formed, which, when by the motion of the telescope is brought 
to coincidence with the wires, as seen in the eye-piece of the instrument, indicates 
the vertical position of the tube of the telescope with great accuracy. 



INSTRUMENTAL ADJUSTMENTS. 279 

SECTION VI. 

®;^e f rausit Clock. 

One of the chief requisites for an astronomical clock is an invariable pendulum, 
for on the regularity of its vibrations the indication of time entirely depends. As 
no single substance has ever yet been discovered, of which a pendulum may be 
made which is not liable to expand and contract by changes in the temperature, 
several materials have been successfully used creating an opposition of expansion, 
being such an arrangement of the materials used, that while some of them increase 
the distance of the centre of oscillation from the point of suspension, others 
shorten the said distance in the same proportion. 

As the astronomical clock indicates 0/i. Om. Os. when the vernal equinox is on the 
meridian, if the clock shows the time to be 3A. 10m., it signifies that it is 3A. lOw. 
since the first point of Aries was on the meridian, and does not specify the hour 
of the day corresponding to the time since the Sun passed the meridian. Astro- 
nomical clocks generally indicate sidereal time ; that is, the time which elapses be- 
tween two transits of a star over the meridian of any place. 

The most approved clock is that called the electric clock. This instrument, 
which is known by the name of the electro-chronograph, is a combination of the 
magnetic clock, Morse's telegraphic register, a break-circuit key, or instrument 
for breaking the magnetic circuit. The magnetic clock was invented by Mr. 
Wheatstone, in 1841. An invention of a similar character was also made by Mr. 
Bond, of the Cambridge (Massachusetts) Observatory. The object of this instru- 
ment is for the determination of the exact period of a transit or other astronomical 
observation, by which longitude may be ascertained to the hundredth or even thou- 
sandth part of a second. The difference of longitude between any two places, is 
determined by observing the period of the occurrence of certain celestial pheno- 
mena, such as eclipses, transits, occultations, &c. In order to insure perfect 
accuracy, the utmost exactitude in regard to time is of the greatest importance. 
The usual practice has been for the observer to note the exact time of a transit or 
other phenomena, by listening to the beats of a clock or chronometer, and if an 
event should occur between two beats, to estimate the fraction of a second. To 
attain any proficiency in this branch, requires a nicety of hearing only to be ac- 
quired by long practice. By the electro-chronograph, the observer can record 
the exact time without taking his eye from the telescope. 

If the toothed or electrical wheel be so adjusted as to let the tilt-hammer rest 
between the teeth during the current second, and be tripped only suddenly at 
every escape, the clock will be made to print lines representing seconds, thus — 

Fiff. 191. 



Or, by reversing the action of the graver, the dots will be the length of the 
spaces in the above figure, thus — 

Fig. 192. 

But one of the sixty teeth is cut off ; consequently, at every revolution of the 
wheel, which is once per minute, a line is produced as below — 



280 BOrVIER'S FAMILIAR ASTRONOMY. 

In this manner are minutes marked on the fillet. Spaces of time equal to five 
minutes and the commencement of an hour are also marked in this vay. 

"With one of these clocks, the difi"erence of longitude between the National Ob- 
seiTatorr at Washington and any other point connected by telegi-aph, may be deter- 
mined in one night so closely, as to show m ichat part of the observatory the obser- 
vations were made. "And thus," in the words of Lieutenant Maury, "this 
problem, which has vexed astronomers and navigatoi-^ and perplexed the world for 
ages, is reduced at once, by American ingenuity, to a form and method the most 
simple and accurate. "While the process is so much simplified, the results are 
greatly refined. In one night the longitude may now be determined with far more 
accuracy by means of the magnetic telegraph and clock, than it can by years of obser- 
vation according to any other method that has ever been tried.'' 

Dr. Locke, of Cincinnati, constructed a clock which enables the observer to 
record his observations by means of electro-magnetism. This clock interrupts the 
electric circuit at each second, and produces breaks which represent the circuit 
on a fillet of paper at the other end of the line. The dashes or lines between each 
break being exactly of the same length, each break represents a second. A wheel, 
consisting of sixty teeth, makes one revolution per minute. The handle of a 
small platinum tilt-hammer, resting on a bed of platinum, is struck by each tooth 
as the wheel revolves ; this raises up the little tilt-hammer, which qtiickly resumes 
its place on the platinum bed. The fulcrum of the hammer and the platinum bed 
rest on a block of wood, and to each is attached one of the wires connected with a 
pole of the battery. By this contrivance, the electric circuit is alternately broken 
and formed by the rising and falling of the little hammer. 

In the "Washington Observatory, the pendulum is connected at its upper extre- 
mity with one of the wires from the battery ; at the lower extremity of the pendu- 
lum is a fine metal point, which at every vibration passses through a globule of 
mercui-y placed in a small metal cup. A wire from the other pole of the battery 
is attached to the metal cup ; so that when the pendulum touches the mercury, the 
circtiit is complete, but is broken the moment the point of the pendulum is no 
longer in contact with it. 

The lines between each break represent a second ; and by an ingenious arrange- 
ment of the machinery, the end of each minute, each five minutes, and of each 
hour, may be recorded accurately. An astronomer at any station on a line of 
several thousand miles in length, may impiint on the register the date of any 
event, by simply tapping upon a break-circuit key. This imprints on the indented 
line a corresponding break-circuit space. 

The clock in its ordinary movements causes the registering machine to mark an 
interrupted time scale on a running fillet of paper, (such as is used in a telegraph 
office.) 



GRADUATED CIRCLES. 281 

CHAPTER VI. 

iraiWEUi €ixtln. 

There are several instruments compreliended under this head, of which the fol- 
lowing are the most prominent: — The Ilural circle, the Transit circle, the Alti- 
tude and Azimuth instrument, the Sextant, and the Repeating circle ; but as our 
limits will not admit of full descriptions in this branch of astronomical science, we 
shall confine ourselves to the three first. 

SECTION I. 

This is an instrument which aflFords means of measuring arcs on the meridian. 
It consists of a circle of brass or other metal ; and if for a large observatory, it 
should be about six feet in diameter. The mural circle is securely fixed to a per- 
pendicular wall, (whence its name,) and is placed accurately in the plane of the 
meridian. The wall is composed of solid masonry, to insure steadiness of position. 
The axis of the circle is horizontal, and inserted into a stone pier, so that the 
plane of the circle may be in the plane of the meridian. The circle is connected 
with the central portion of the wheel by spokes or radii. These radii are com- 
posed of hollow cones, which are braced together by a set of bracing bars situated 
about midway between the centre and circumference of the wheel. The axis of 
the wheel is a cone of brass nearly 4 feet long, and 7 inches in diameter in the 
largest or front part of the cone, diminishing to about 3^- inches at the other ex- 
tremity of the axis. 

The telescope has a focal length of 6 feet 2 inches, the aperture is 4 inches, and 
the magnifying power about 150. At the focus of the telescope are vertical wires, 
which are illuminated by a diagonal reflecting plate, fixed in the middle of the 
tube, which receives the light through the circular aperture, exactly in a line with 
the centre of the circle, as shown in the following drawing. 

A lantern at four or five feet distance, placed in the line of the axis, throws 
light on the field of view equal to the brightness of daylight, which light may be 
regulated and modified by means of colored glasses. 

The limb of the circle consists of two rings, the inner one having its plane 
parallel, and the outer one its plane perpendicular, to the plane of the circle, so that 
when their edges are united, their section may form the figure of the letter T. 

The graduation of this instrument is made on the broad surface of the exterior 
ring, which is at right angles to the plane of the circle. The divisions are made 
upon a narrow ring of white metal, composed of four parts of gold to one of palla- 
dium ; and the figures which count the degrees are engraved upon a similar ring 
of platina ; neither of these metals tarnish in the least, which renders frequent 



*The description of this instrument, as well as of many others contained in this work, is taken 
from that admirable treatise, by the Rev. Dr. Pearson, entitled Practical Astronomy, 2 vols. 4to, 
1 vol. plates. 



282 



BOUVIER S FAMILIAR ASTRONOMY. 



cleaning of the surface unnecessary, as it would in time wear out the fine divisions. 
The divisions are by lines, not dots, and the degrees are cut into twelve parts or 
spaces of S'' each, and are numbered from the pole southward round to the same 
pole again — viz, from 0° to 360°. The 5'' spaces are subdivided by the microscopes 
to single seconds, and a division representing this quantity on the micrometer 
head may easily be estimated to the tenth of a second. 

Fis. 193. 




WASHINGTON MURAL CIRCLE. 
The mural circle at the observatoi-y at Washington, of which the accompanying 
figure is a representation, is a circle of brass, five feet in diameter, having twelve 
T-barred spokes, which support a brass rim or tire four-tenths of an inch thick 
and two inches broad. The graduations are made in the periphery of this rim by 
lines parallel with the axis of the circle. The rim has two bands upon it — one in 
platina, on which the degrees are numbered from 1° to 360° ; the other band is 
of gold, on which the subdivisions are made. These subdivisions are for every 5' 



GRADUATED CIRCLES. 283 

of arc, the divisions corresponding to a degree being designated by a large dot ( • ) 
at one end ; those corresponding to 15^ by a smaller dot (.) ; those to 30^ by two 
dots (:); and those to 45^ by three dots (.-.). 

In the figure, A B C D E F are reading microscopes, the wires of which for read- 
ing the instrument form with each other acute angles, which the divisions on the 
circle are made to bisect. The telescope is also furnished with a micrometer 
thread, which is parallel to the horizontal wire of the fixed diaphragm. The head of 
this micrometer is divided into 100 parts ; readings therefore may, by subdivision, 
extend to thousandths of a revolution. The telescope is attached to an axis con- 
centric with that of the circle, upon which it may be made to revolve independently 
of the circle. M M M M M are clamps with tangent screws, for slow motion. 
R R are friction rollers, supported by the flat rods r r, which are kept in action 
by the levers 1 1, having their compensating weights at the other end, to receive 
which weights a hole is cut in the top of the pier. T is the telescope, m the micro- 
meter head ; V is a milled-headed screw, working two rods with rachets, for regu- 
lating the illumination of the field by means of a square cut-ofi" at the end of the 
rods ; o o and o o are screws for securing the telescope at each end to the circle ; n 
and n are scores cut for the more convenient reading of the microscopes C and E ; 
P is the fixture for holding the plummet, which is suspended by a fine silver wire ; 
and j9j9 the ghost apparatus for levelling. H H is the bench for the artificial hori- 
zon, wrought out of the solid stone ; between the figures 1 and 2 it is 9 inches 
thick, 12 inches from 2 to 3, and also 12 from 3 to 4. hh are steps on which the 
trough is placed for low stars ; thus, an observation may be made by reflection 
without removing the basin of mercury from its bench. Nearly in the same plane 
with the wires of the reading microscopes, and visible through the eye-piece of 
each, is a scale like a saw, the distance between the teeth of which corresponds 
with the distance between the threads of the micrometer screw. Every fifth notch 
is deeper than the rest ; the middle of the scale or zero-point of the microscope is 
designated by a round hole ; and the notch which denotes the number of entire 
revolutions corresponding to a reading, is indicated by a little index or pointer 
attached for the purpose. 

In the following figures the different parts of the reading microscope will be 
represented: a is a spider-line micrometer, with one screw; b [figs. 194 and 196) 
is a tube, two inches long, having a screw on the exterior face and one on 
the interior; it is screwed to the lower plate of the micrometer, and at right 
angles to it ; c is the cock to which a short tube of half an inch in length is fixed, 
that admits the tube 6 just to pass into it; dd are two circular nuts, with milled 
heads ; the use of these nuts is to fix the tube b in any required position ; e is 
the cell that receives the eye-piece, and screws into the outer plate of the micro- 
meter; ^ is a cone of brass, holding the object-lens at its inferior end, the other 
end being screwed into the body of the tube b; h (figs. 194 and 196) is a small 
cell that holds the object-lens; i is a third circular nut, with a milled head, by 
means of which the distance of the object-lens may be graduated. The two pieces 
of^^r. 195 lie over one another in the oblong hos., fig. 197 ; one having the scale 
of notches on the edge of its large opening, and the other carrying the crossed 
spider lines, immediately above the said scale. As they both fit the sides of the 



284 



BOUVIER S FAMILIAR ASTRONOMY. 



box, they are thus kept parallel to one another when moved separately over each 
other. The fork, having its prongs connected by an end-piece k, is moved by 
means of the milled nut and divided head, seen 'va.fig. 194; and the crossed lines 
and included piece of wire forming an index to the notches, travel over the scale 
m, the place of -svhich is adjusted by a small screw n passing through an X spring, 
as shown 'va.fig. 197. A strong pin rises from the tmder side of the oblong box o, 
{fig. 197,) and passing within the fork and slit made at^ through the plate carrying 
the notched scale, presents its upper end to the small spiral spring which, acting 
in opposition to the micrometer's screw I, prevents any loss of motion in turning 
it. The fixed piece q {figs. 194 and 196) is the index to the divided head r, turn- 
ing with the screw, and showing exact seconds. 



T\z. 194. 




Fior. 195. 









N 


(T^ 








\m 




^ 


11 ^ 




. 




\p 




r 




\ 


1 


. 






5 







Fig. 197. 



i 



ff-^ 



^-■■■■r ^ 


r^^r^-^ 




^ 


= 


If 


fin 


^^ 


i 




T 




;^^ 





SECTION II. 

tht transit Circle. 

The object of the transit circle is to give the right ascensions as well as the de- 
clinations of stars at the same time. This instrument consists of two circular 
rings or wheels, the spokes of which are formed of two sets of hollow cones ; the 



GRADUATED CIRCLES. 285 

two wheels are united together by various bars crossing each other. Each of these 
wheels is divided or graduated into four quadrants, by spaces of 5^. The axis of 
the wheels is supported by two stone piers, which have their perpendicular and 
parallel faces separated about 27 inches. To the interior face of each pier, 
just below their summits, is a strong horizontal bar of brass made fast, the ex- 
treme ends of which are turned up so as each may hold a pair of microscopes for 
reading the divisions at the opposite ends of the horizontal diameter of the circle, 
to which they are capable of being adjusted. The zenith distance or altitude may 
be read on the circles, and at the same time a transit may be taken in the usual 
way, by the system of spider lines that are fixed in the common focal point of 
the object-glass and eye-piece. Hence, it is obvious that, by the proper use of 
this instrument, a star or other heavenly body may have its place referred both 
to a circle of declination and to the equator, at the same instant. 

SECTION III. 

giltiluijc anb gi^imutlj |nstriimenf. 

The chief use of this instrument is to find the altitude and azimuth of a star ; 
or, to observe the Moon when in those portions of her orbit too near to the Sun to 
be observed on the meridian ; her proximity to that luminary for two or three days 
before and after her conjunction, rendering her invisible when on the meridian. 

This instrument is composed of a horizontal and a vertical axis, to each of 
which is attached a graduated circle. That circle which moves in a horizontal 
plane is the azimuth circle ; and that moving in a vertical plane is called the ver- 
tical circle. The vertical circle consists of two circles connected together by small 
brass bars. The side of one of the circles is inlaid with a ring of silver, on which 
the graduation is made. The azimuth circle, or that which moves in the plane of 
the horizon, is graduated and read off by means of two reading microscopes. The 
telescope is attached to the horizontal axis, somewhat like that of the transit in- 
strument. The vertical circles are placed close to the telescope, and are read by 
means of two microscopes attached to two pieces of metal fastened near the top 
of one of the pillars which support the horizontal axis of the instrument. The 
spider-lines in the telescope are illuminated after the manner of the transit in- 
strument. 

To adjust the instrument, it is necessary to level the horizontal or azimuth circle, 
and also the axis of the telescope, as in the transit instrument. When the tele- 
scope is directed north and south, the meridianal point on the azimuth circle may 
be determined by observing a star at equal altitudes from the meridian, east or 
west, and determining the point exactly between the two observed azimuths. The 
instrument may be adjusted to the meridian in the same manner as for the transit 
instrument. The horizontal point of the altitude circle is its reading when the 
axis of the telescope is horizontal, and may be found as with the mural circle, by 
alternate observations of a star direct, and reflected from the surface of mercury. 

There are many other astronomical instruments in use in observatories, besides 
those above described, which the student may find fully explained and elegantly 
illustrated in that great work, "Practical Astronomy," by the Rev. W. Pearson. 



286 



BOUVIER S FAMILIAR ASTRONOMY. 



Many of the modern instruments may also be found in the works on Practical 
Astronomy by Professor Loomis, Dr. Dick, and others. 

The grand attainments in the various branches of astronomy have not been ac- 
quired by any one individual, but are the combined efforts of all former ages ; 
thus contributing to our mental instruction and moral improvement, as if seen 
vrith our own eyes, heard vrith our own ears, and handled with our own hands, 
from the earliest periods on record. From age to age our mass of knowledge ia 
continually increasing, as well as our facilities for further investigations. Lee 
each one, therefore, contribute his mite to the general stock, for the benefit of 
those who are to come after us. 



This figure represents a cast-iron 
equatorial telescope, with 4j inches 
ajierture and 72 inches fo.^us. Price, 
$300. Manufactured by Henry Fitz, 



Esq.. of Xew York, who will furnish 
every variety of telescope, from S125 
to $6000. These instruments haTS 
a very high reputation. 




PAET Y. 



The following problems are designed to familiarize the student 
with the principles of Geography and Uranographj, in connection 
with the phenomena of the heavenly bodies. 



CHAPTER 5. 

|rj(jMmg m Hit i^rr^strial itote, 

PKOBLEM I. 

To find the latitude and longitude of any place. 

Rule. — Bring the given place to the graduated side of the 
brass meridian ; the degree of the meridian directly over the place 
shows the latitude, and the degree on the equator cut by the brass 
meridian shows the longitude.* 

EXAMPLES. 

1. What is the latitude and longitude of Washington ? 

Ans. Lat. 38° 53' north, Ion. 77° 15' west. 

2. What is the latitude and longitude of the Cape of Good Hope ? 

Ans. Lat. 33° 56' south, Ion. 18° 23' east. 
Required the latitudes and longitudes of the following places : 



3. Boston. 

4. Canton. 

5. London. 

6. Cairo. 

7. Cincinnati. 



8. Glasgow. 

9. Gottingen. 

10. Paris. 

11. San Francisco. 

12. Rome. 



PROBLEM II. 

To find any place on the globe, the latitude and longitude of 
which are given. 

Rule. — Bring the given longitude to the meridian, then under 
the given latitude, on the brass meridian, will be found the place 
required. 



* The longitudes are reckoned from the meridian of Greenwich. 

287 



288 bouvier's familiar astronomy. 



EXAMPLES. 



1. Find the place which is situated in lat. 57° 8' N., and Ion. 
2° 10' W. 

2. What place is situated in lat. 39° 57' N., and Ion. 75° 7' W. ? 
Find those places having the latitudes and longitudes as follows: 



3. 


Lat. 41° 1' N. 


Lon. 28° 50' E. 


4. 


53° 23' N. 


6° 20' W. 


5. 


19° O'S. 


174° O'W. 


6. 


43° 17' N. 


5° 22' E. 


7. 


22° 53' S. 


43° 12' W. 


8. 


Find all those places which have no latitude. 


9. 


Find those places 


which have no longitude. 
PROBLEM III. 



To find the difference of latitude hetiveen any two places. 

Rule. — Find the latitude of both places, and the number of 
degrees between them reckoned on the brass meridian will be the 
difference of latitude. Or find the latitudes of both places, and 
if they be of the same name — that is, both north or both south — 
their difference is the difference of latitude ; but if one is north 
latitude and the other south, their sum will be the difference of 
latitude. 

examples. 

1. AVhat is the difference of latitude between Washington and 
Berlin ? Ans. 13° 38'. 

2. What is the difference of latitude between London and the 
Cape of Good Hope ? Ans. 85° 27'. 

What is the difference of latitude between the following places ? 

3. Lisbon and Rio Janeiro. 

4. St. Petersburg and Calcutta. 

5. Cape Horn and Cape of Good Hope. 

6. Isthmus of Darien and Isthmus of Suez. 

7. Quito and Philadelphia. 

8. North Pole and the South Pole. 

PROBLEM lY. 

To find the dfference of longitude between any two places. 

Rule. — Bring one of the given places to the brass meridian, 
and find what degree of the equator is cut by it ; then bring the 
other place to the brass meridian, and the number of degrees 
counted on the equator the shortest distance between the two, 
will show the difference of longitude. Or, after finding the longi- 
tudes of both places, if they be of the same name — that is, if b6th 
be east or both be west — their difference is the difference of 



PROBLEMS ON THE TERRESTRIAL GLOBE. 289 

longitude ; but if they be of different names, their sum will give 
the difference of longitude. 

EXAMPLES. 

1. "What is the difference of longitude between Bremen and 
New York ? Ans. 65° 11'. 

Find the difference of longitude between the following places : 

2. Cadiz and Cincinnati. 

3. Savannah and Alexandria. 

4. Athens and Quebec. 

5. St. Petersburg and Nova Zembla. 

6. Paris and Baltimore. 

PROBLEM V. 
To find the difference of time between two places. 
Rule. — Find the difference of longitude as in the last problem, 
and divide the number of degrees thus ascertained by 15. 

EXAMPLES. 

1. What is the difference of time between Philadelphia and 
Liverpool ? Ans, 4:h. 48m. 52s. 

Therefore, when it is noon at Liverpool, it is 4^. 48m. 52s. before noon, or 7h. 11m. 8s. 
in the morning at Philadelphia; because Liverpool lies east of Philadelphia, the sun 
rises at that place first. 

2. When it is 10 o'clock in the morning at Paris, what is the 
time at Philadelphia ? 

Ans. 49m. and 56s. past 4 in the morning. 

3. When it is noon at Montreal, what time is it at Canton ? 

4. What is the difference of time between London and New 
Orleans. 

5. When it is 6 o'clock A. M. at Quito, what time is at Con- 
stantinople ? 

6. When it is midnight at Calcutta, what time is it at Boston ? 

7. When it is noon at Paris, what time is it at San Francisco ? 

PROBLEM VI. 

To find all those places which have the same latitude as any 
given place. 

Rule.— -Bring the place to the brass meridian, turn the globe, 
and all those places which pass under the same degree of the 
brass meridian as the given place have the same latitude. 

EXAMPLES. 

1. What places have the same latitude as St. Louis ? 

Ans. Frankfort, Ky. ; Portsmouth, Ohio; Pico, one of the 
Azores ; Murcia, in Spain ; Malta ; Palermo ; Pekin ; Kingkitao ; 
and Shanday. 



290 bouvier's familiar astronomy. 

Find all those places which have the same latitude as the fol- 
lowing 



2. Charleston. 

3. Quito. 

4. San Francisco. 

5. Truxillo. 



6. Mount Heckla. 

7. Sierra Leone. 

8. London. 



PROBLEM VII. 



To find those places which have the same longitude as any 
given place. 

Rule. — Bring the place to the brass meridian; those places 
situated under the same edge of the brass meridian have the 
same longitude. 

examples. 

1. What places have the same longitude as Boston ? 

Ans. Whale Sound and Quebec, in North America ; and Coro, 
Cuzco, Copiapo, Totora, Coquimbo, and Valparaiso, in South 
America. 

2. What places have the same longitude as New Orleans ? 



5. City of Mexico? 

6. Mecca? 



3. Calcutta? 

4. St. Petersburg ? 

Those places which have the same longitude have the same hour of the day. 

7. What places have their noon the same time as at Paris ? 

8. What places have the same time as Rome ? 

PROBLEM VIII. 

To find the antoeci of any place. 

Rule. — Bring the given place to the brass meridian and find its 
latitude ; then under the same meridian, and in the same degree 
of latitude in the opposite hemisphere, you will find the antoeci. 

EXAMPLES. 

Required the antoeci of the following places : 

1. New York. Ans. Valdivia, in South America. 

2. Juan Fernandez. Ans. Charleston. 

3. Boston. 

4. Azof. 

The antoeci have the same hours, but contrary seasons of the year ; thus, it is noon 
with both at the same time, but winter with one when it is summer with the other. 

5. Required those places whose seasons are directly opposite to 
those of Quebec. 



6. Malta. 

7. Bermudas. 



8. Potosi, South America. 

9. Rome. 



PROBLEMS ON THE TERRESTRIAL GLOBE. 291 

PROBLEM IX. 

To find the antipodes of a given place. 

Rule. — Bring the place to the brass meridiaii, mark the lati- 
tude, and set the index to 12 ; turn the globe till the index has 
passed over 12 hours, then count as many degrees of latitude 
towards the contrary pole as are equal to the latitude of the place ; 
this point will be the antipodes of the given place. Or, bring the 
given place to the horizon, and the opposite point of the horizon 
will be the antipodes. 

EXAMPLES. 

1. Find the antipodes of Oporto. 

Ans, Cook Straits, island of New Zealand. 

2. A ship in the Indian Ocean found its latitude was 9° S., and 
longitude 101° E. from Greenwich : where are its antipodes ? 

Ans. Panama. 

3. Where are the antipodes of the Friendly Isles ? 

4. Suppose a line were to be drawn through the centre of the 
Earth from a point in the Southern Ocean, lat. 40°' S., and Ion. 
105° E. from Greenwich: what place on the surface would it 
touch on the opposite side ? 

5. What places on the Earth are the seasons and hours con- 
trary to those of Washington City ? 

PROBLEM X. 

To find the perioeci of a given place. 

Rule. — Bring the given place to the brass meridian, note the 
latitude, and set the index to 12 ; turn the globe till the index 
has passed over 12 hours. The place under the same degree of 
latitude is the perioeci required. 

examples. 

1. Required the perioeci of Merida, in Yucatan. 

Ans. The mouths of the Ganges. 

2. What place has the same seasons as Astoria, but contrary 
hours ; that is, when it is noon at one, it is midnight at the other 
place? Ans. The northern part of the Sea of Aral. 

Required the perioeci of the following places : 

3. Cape May. 6. Constantinople. 



4. St. Petersburg. 



7. Rome. 



5. Buenos Ayres. | 8. Vera Cruz. 

PROBLEM XI. 

To find the distance between two places. 

Rule. — When the distance is less than 90°, lay the quadrant 



292 bouvier's familiar astronomy. 

of altitude over both places, so that the division marked may 
be over one of them ; the degree cut by the other place will show 
the distance in degrees, which can be reduced to miles by multi- 
plying them by 69J, because 69|- miles make one degree. But 
when the distance is more than 90°, subtract the distance from 
180, and the remainder will be the distance required. 

EXAMPLES. 

1. Required the distance between Philadelphia and Calcutta. 

Ans. 7831 miles. 

2. Philadelphia and Liverpool. Ans. 50°, or 3484 miles. 

3. Philadelphia and New Orleans. 

4. Philadelphia and San Francisco. 

5. What is the length of the United States from the northern 
point of Maine to Florida Keys ? 

6. What is the distance between Paris and the Crimea ? 

7. London and St. Petersburg. 

8. Rome and Jerusalem. 

9. Madra's and Lisbon. 

10. How many miles will be travelled over in the following 
route? — From Washington City to Cincinnati, Nashville, St. Louis, 
Chicago, Niagara, Albany, Boston, and Philadelphia. 

PROBLEM XII. 

The hour being given in any place, to find what hour it is in 
any 2^(^'^t of the ivorld. 

Rule. — Bring the given place to the meridian, and set the 
index to the given hour ; then turn the globe till the other place 
comes to the meridian, and the index will show the time required. 

examples. 

1. When it is noon at Washington City, what o'clock is it at 
London ? Ans. 5h. 7m. p. m. 

The above problem can be solved without the globe, thus : — 
Find the diiference of longitude between the two places in degrees, 
and reduce it to time by multiplying the degrees and minutes by 
4; the product will be minutes and seconds. Or divide the num- 
ber of degrees by 15 for the answer in hours, and if there be a 
remainder, multiply it by 4 for the minutes. 

The difference of longitude in time will be the difference of time 
between the two places. If the place at which the time is required 
be east of the place at which the time is given, the time required 
will be later than the given time ; but if the place at which the 
time is required be west of the place at which the time is given, 
the required time will be earlier than the given time. 

In the above question it is required to know what time it is at 



PROBLEMS ON THE TERRESTRIAL GLOBE. 293 

London when it is 12 o'clock at Washington. Now, as London, 
the place at which the time is required, is east of Washington, the 
time required must be later than 12 o'clock, the given time. 
The problem is solved bj calculation, thus : 
770 3/ w = Ion. Washington. 
9 W = Ion. London. 

76 54 = difference of Ion. 
4 



60) 307 36 

5A. 1m. 36s. the difference of time between the two places. 
Consequently, when it is noon at Washington, it is 5A. 7m. 36s. P. M. 
at London, because it lies east of Washington ; and therefore the 
time must be later. 

3. When it is 6 A. M. at Paris, what time is it at Constantinople ? 

4. When it is midnight at New York, at what places is it noon ? 

5. When it is 4 A. M. at Charleston, what time is it at Calcutta? 

6. What hour is it at Boston, when it is 10 o'clock A. M. at 
London ? 

GIVEN TIME. PLACE AT WHICH TIME IS GIVEN. PLACE WHERE TIME IS REQUIRED. 

7. 7 A. M. St. Louis. Berlin. 

8. 5 p. M. San Francisco. Philadelphia. 

9. Noon. Quito. Borneo. 

10. 11 p. M. Gottingen. Algiers. 

11. 8 p. M. Ispahan. Fort Churchill. 

12. 9 A. M. . Corinth. Truxillo. 

PROBLEM XIII. 

The day of the month being given, to find the Suns place in 
the ecliptic. 

Rule. — Find the given day in the circle of months on the 
wooden horizon, and opposite to it in the circle of signs are the 
sign and degree which the Sun occupies on that day. Find the 
sign and degree on the ecliptic on the globe, and bring the de- 
gree of the ecliptic thus found to that part of the brass meridian 
which is numbered from the equator to the poles ; then that degree 
of the meridian which is over the Sun's place is the declination 
required. Or, 

By the Analemma. 

Bring the analemma to the brass meridian, and the degree cut 
on it directly above the given day of the month is the Sun's de- 
clination. Bring that part of the ecliptic which corresponds with 
the given day under this degree of the Sun's declination, and 
that point will be the Sun's place in the ecliptic, or his longitude 
for that day. 



294 bouvier's familiar astronoxMY. 



EXAMPLES. 



1. Kequired the Sun's longitude and declination on the 12th 
of May. Ans. 21° 45' ; in b' dec. 18° N. 

What is the Sun's longitude and declination on the following 
days ? — 



2. June 30. 

3. May 19. 

4. January 1. 

5. Christmas-day. 



6. March 3. 

7. November 12. 

8. February 22. 

9. August 6. 



PROBLEM XIV. 



To rectify the globe for the Suns place on a given day. 

Rule. — Find the Sun's declination for the given day, and bring 
his place to the brass meridian ; elevate the pole which is of the 
same name as the declination ; that is, if the declination is north, 
elevate the north pole, and if it be south, the south pole. The 
pole should be elevated as many degrees as are equal to the 
declination. 

When the globe is thus rectified, all those places above the wooden horizon will be in 
the enjoyment of day j and those places below it will be in darkness, or night. 

examples. 

1. Rectify the Globe for the Sun's place on the 1st of June. 

Ans. The Sun's declination on the 1st of June is 22° N. ; there- 
fore the north pole must be elevated 22°, and all those places 
situated within 22° of the north pole must have continual day, 
while to those situated within 22° of the south pole there is no 
sunlight. . 

2. Rectify the globe for the Sun's place on the 22d of December. 
Ans. The Sun's declination on the 22d of December is 23° 30' 

S. ; therefore the south pole must be elevated ; and to all places 
situated within the Arctic Circle, which is 23° 30' of the north 
pole, the Sun does not rise, while to those places situated within 
the Antarctic Circle, he never sets. 

3. Rectify the globe for the 21st of September. 

4. For the 1st of May. 

5. For the 23d of March. 

6. For the 16th of August. 

problem XV. 

To find the time of the Sun's rising and setting, and the length 
of the day and night at any place. 

Rule. — Elevate the pole according to the latitude of the place, 
bring the Sun's place to the meridian, and set the index to 12. 



PROBLEMS ON THE TERRESTRIAL GLOBE. 295 

Then bring the Sun's place to the eastern horizon, and the index 
will show the time of the Sun's rising ; when the Sun's place is 
brought to the western horizon, it shows the time of his setting. 

EXAMPLES. 

1. What time does the Sun rise and set in New York on the 
10th of May ? Ans. The Sun rises at 5, and sets at 7. 

The length of the day is found by multiplying the time of sunsetting by 2, and the 
length of the night by doubling the time of sunrising. Also, the length of the night 
taken from 24 will give the length of the day, and vice versa. Moreover, half the length 
of the day gives the time of sunsetting, and half the length of the night the time of 
sunrising. 

The length of the day at New York on the 10th of May is 14 hours, and the length 
of the night 10 hours. 

2. What time does the Sun rise and set at London on the 1st 
of December ? and what is the length of the day and night ? 

Ans. The Sun rises at 8h. 40m., and sets at Sh. 20m. 

3. At what time does the Sun rise and set at every place on 
the globe on the 21st of March and on the 23d of September ? and 
what is the length of the day ? 

Ans. It rises at 6 and sets at 6, and the day is 12 hours long. 
Which is the longest day at the following places ? — 



4. London. 

5. Quito. 
Required the shortest day at the following places : 



Terra del Fuego. 
Melville Island. 



8. Disco Island, in Davis 

Strait. 

9. Stockholm. 
10. Vienna. 



11. Rio Janeiro. 

12. Pekin. 

13. Astracan. 



PROBLEM XVL 

To find the Suns meridian altitude for any day. 

Rule. — Elevate the globe for the latitude of the given place ; 
find the Sun's place for the given day, and bring it to the brass 
meridian. Fix the quadrant of altitude on the zenith, and bring 
it over the Sun's place ; then the degree upon the quadrant cut; 
by the Sun's place will be its meridian altitude. 

EXAMPLES. 

1. What is the Sun's meridian altitude at Washington City on 
the 21st of June ? Ans. 74° 35^ 

2. What is the Sun's meridian altitude at Charleston on the 
shortest day in the year ? 

3. What is the Sun's meridian altitude at Berlin on the 4th of 
July? 



296 



BOUVIER S FAMILIAR ASTROXOMY. 



4. "What is the Sun's meridian altitude at the Cape of Good 
Hope on the 10th of February ? 

5. What is the Sun's meridian altitude at Melville Island on 
the 1st of July ? 

What is the Sun's meridian altitude at the following places ? — 

6. Quito, on the 8th of May. 

7. Pernambuco, on the 12th of June. 

8. Truxillo, on the loth of September. 

9. Samarcand, on the 8th of August. 

10. Quebec, on the 4th of January. 

11. Aleppo, on the 3d of March. 

12. Cairo, on the 19th of December. ' 

PROBLEM XVII. 

To find the Suns altitude for any liour^ having the latitude 
and the day of the month given. 

Rule. — Rectify the globe for the latitude, screw the quadrant 
of altitude over the zenith, bring the Sun's place for the given 
day to the brass meridian, and set the index to 12. Turn the 
globe till the index points out the given hour, bring the graduated 
edge of the quadrant over the Sun's place, and the degree cut on 
it will be the Sun's altitude. Or, elevate the pole to the Sun's 
declination, screw the quadrant of altitude in the zenith, bring the 
given place to the brass meridian, and set the index to 12. If 
the given hour be in the forenoon, turn the globe westward ; but 
if it be in the afternoon, turn it eastward, as many hours as the 
time is before or after 12 ; bring the quadrant of altitude over 
the given place, and the degree cut on it will be the Sun's altitude. 

EXAMPLES. 

1. What is the altitude of the Sun at St. Petersburg: on the 



21st of March, at noon ? 



Ans. 60°. 



2. Required the altitude of the Sun at the City of Mexico on 
December 5, at 7 A. M. ? Ans. 6°. 

3. What is the Sun's altitude at Sydney on the 12th of April, 
at 11 A. M. ? 

4. What is the Sun's altitude at St. Helena on the oth of August, 
at 3 P. M. ? 

5. What is the Sun's altitude at Spitzbergen, June 21, at mid- 
night ? 

6. What is the Sun's altitude at Boston on September 3, at 
4 p. M. ? 

PROBLEM XYIII. 
To find all those jjlaces ivhere the Sun has the same altitude 
as any given place^ at any given time. 

Rule. — Find where the Sun is vertical at the o^iven time — that 



PROBLEMS ON THE TERRESTRIAL GLOBE. 297 

is, find his declination — bring the given place to the brass meri- 
dian, and set the index to the given hour ; if the hour be in the 
forenoon, turn the globe westward ; and if in the afternoon, turn 
it eastward, until the index points to 12. Mark the places exactly 
under the Sun's declination, and then place the degree of the 
quadrant of altitude marked 0° over the place thus found : all 
those places the same distance from it as the given place will be 
the places required. Those places situated at the same distance 
from it as the given place may be found by holding the part of the 
quadrant marked 0° on the place found, and then turning it round : 
the places which come under the same degree of the quadrant as 
the given place will be the same distance from that found. 

EXAMPLES. 

1. When it is 10 o'clock in the morning at Washington City on 
the 5th of May, required all those places where the Sun, at that 
moment, will have the same altitude as at Washington. 

Ans. The place where the Sun will then be vertical is lat. 16° 
N., Ion. 47° W. from Greenwich ; and those places which are at the 
same distance from it as Washington, are — Quito ; Sierra Leone ; 
Providence, R. I. ; Baltimore ; Columbia, S. C. ; St. Mary's, Fla. 

2. When it is 4 o'clock in the afternoon at Paris, on the 10th 
of August, find all those places where the Sun will have the same 
altitude as at Paris. 

3. Find all those places where the Sun will have the same alti- 
tude as at New Orleans on the 21st of September, at 11 o'clock. 

4. Name all those places where the Sun will have the same 
altitude as at Mecca on the 4th of July, at 7 o'clock, A. M. 

PROBLEM XIX. 

Heaving the Suns meridian altitude, to find the latitude of the 
place. 

Rule. — Bring the Sun's place to the brass meridian, and mark 
his declination. Note as many degrees on the brass meridian 
from this point as is equal to the given altitude — reckoning towards 
the south, if the Sun be south of the observer, and towards the 
north, if he be north of the observer. Bring the degree where 
the reckoning ends to coincide with the horizon, and the number 
of degrees the elevated pole is from the horizon is the latitude re- 
quired. Or, by calculation, thus : — Subtract the given altitude 
of the Sun from 90° ; the remainder will equal the zenith dis- 
tance, which is north if the zenith be north of the Sun, or south 
if the zenith be south of it ; take the Sun's declination from the 
table for the given day, and observe whether it be north or south ; 
then if the zenith distance and declination be both north or 



298 bouvier's familiar astronomy. 

both south, add them together ; but if one be north and the other 
south, subtract the lesser from the greater, and the sum or differ- 
ence will be the latitude of the same name with the greater. 

examples. 
1. On the 8th of October the meridian altitude of the Sun was 
found to be 20° S., the observer being north of the Sun : required 
the latitude of the place of observation. Ans. 64° 14' N. 

Bi/ Calculation. 
90° 0' 
20 



70 zenith distance N. 
Subtract 5 46 Sun's declination S. 

64 14 N. lat. 

2. What is the latitude of that place at which the Sun's meri- 
dian altitude on the 19th of October was found to be 8° 20' S. ? 

Ans. 71°47'N. 

3. What is the latitude of that place at which the Sun's meri- 
dian altitude on the 30th of May was observed to be 49° 25' N. ? 

Ans. 18° 45' S. 

4. The Sun's meridian altitude on the 10th of May was ob- 
served to be 40° S. : what was the latitude of the observer ? 

5. On the 12th of July the Sun's meridian altitude was ob- 
served to be 50° 30' N. : what was the latitude? 

6. The meridian altitude of the Sun was observed to be 32° 15'" 
S. on the 21st of March : what was the latitude ? 

PROBLEM XX. 

To find when the Sun is due east or west, the latitude of the 
place and the day of the month being given. 

Rule. — Elevate the globe for the latitude, bring the Sun's place 
to the meridian, and set the index to 12. Screw the quadrant of 
altitude on the zenith. If the Sun's declination and the latitude 
be both north or both south, bring the quadrant to the eastern 
point of the horizon ; but if the Sun's declination and the latitude 
be not of the same name — that is, if one be north and the other 
south — bring the quadrant of altitude to the tvestern point of the 
horizon. Turn the globe till the point of the ecliptic denoting the 
Sun's place come to the edge of the quadrant, and the index will 
show the time required. Subtract the hour when the Sun is due 
east, from 12, for the time when he is due west, and the reverse. 

examples. 

1. When is the Sun due east and west at Tripoli, August 15 ? 

Ans. E. 7A. 10m. ; W. 4A. 50m. 



PROBLEMS ON THE TERRESTRIAL GLOBE. . 299 

2. When is the Sun due east at Montreal on the 5th of July ? 
and at what hour will he be due west ? 

Ans. E. 8^. 10m. ; W. U. 50m. 

3. At what hours will the Sun be due east and west at London 
on the 21st of June ? 

4. At what hours will the Sun be due east and west at Gibral- 
tar on the 21st of September ? 

5. At the Cape of Good Hope, February 1 ? 

6. At Prince of Wales Island, north-west coast of North Ame- 
rica, on the 1st of February ? 

7. When will the Sun be due west at Cincinnati on the 20th 
of October ? 

PROBLEM XXI. 

To find on what two days of the year the Sun is vertical at any 
given place in the torrid zone. 

Rule. — Bring the given place to the brass meridian, and mark 
its latitude. Then turn the globe on its axis, and observe the 
two points of the ecliptic which pass under that latitude. Find 
those points of the ecliptic in the circle of signs on the wooden 
horizon, and exactly opposite to them in the circle of months will 
be found the days required. Or, having found the latitude of the 
place, bring the analemma to the meridian, upon which, exactly 
below the latitude, will be found the days required. 

examples. 

1. On what two days will the Sun be vertical at Cape Verd ? 

Ans, May 1, and Aug. 11. 

2. On what two days will the Sun be vertical at Cuzco, in 
Peru? Ans. Feb. 13, and Oct. 28. 

On what days will the Sun be vertical at the following places ? — 



3. Borneo. 

4. Quito. 

5. Bio Janeiro. 

6. Guadaloupe. 

7. Yera Cruz. 



8. St. Helena. 

9. Owyhee. 

10. Seringapatam. 

11. Batavia. 



PROBLEM XXII. 



The day being given, at any place not in the frigid zones, to 
find what other day of the year is of the same length. 

Rule. — Find the Sun's place in the ecliptic for the given day, 
bring it to the brass meridian, and observe the degree above it ; 
turn the globe on its axis, and observe what other point of the 
ecliptic falls under the same degree of the brass meridian : find 



300 . bouvier's familiar astronomy. 

this degree of the ecliptic marked on the wooden horizon, and ex- 
actly opposite to it will be the day of the month required. 

By the Analemma, 
Bring the given day on the analemma to the meridian, and on 
the other half of the analemma will be the day required. 

examples. 

1. What day of the year is of the same length as the 30th of 
January? Ans. November 11. 

2. What day is of the same length as the 25th of April ? 

Ans. 18th of August. 

3. What day is of the same length as the 10th of November ? 

4. If the Sun rise at Philadelphia on the 5th of August at 5 
o'clock in the morning, what other day will it rise at the same 
hour ? 

5. If the Sun is vertical at any place on the 12th of June, how 
many days must elapse before he is again vertical at that place ? 

6. If the Sun rise at Gottingen at 4 o'clock in the morning on 
the 20th of July, on what other day will he rise at the same hour ? 

PROBLEM XXIII. 

The day and hour being given for any place, to find ivhere the 
Sun is then vertical. 

Rule. — Find the Sun's declination, bring the given place to 
the brass meridian, and set the index to 12. If the given time 
be before noon, turn the globe westward as many hours as it 
wants of noon ; but if it be past noon, the globe should be turned 
eastward as many hours as the time is past noon : the place exactly 
under the degree of the Sun's declination will be that which is 
required. 

EXAMPLES. 

1. When it is 2 o'clock in the morning in Washington on the 
28th of July, where is the Sun at that instant vertical ? 

Ans. At Poonah, in India. 

2. When it is half-past seven at Jerusalem on the 24th of Oc- 
tober, where is the Sun then vertical ? 

Ans. At Lima, in Peru. 

3. When it is 9 A. M. at Dublin, February 12, where is the Sun 
vertical ? 

4. At London, December 21, at 2 o'clock a. m. : where is the 
Sun then vertical ? 

5. When it is Ih. 5Sm. at Amsterdam on the 30th of April, 
where is the Sun vertical ? 



PROBLEMS ON THE TERRESTRIAL GLOBE. 801 

6. At the Cape of Good Hope, when it is lA. 37m. on the 6th 
of November, where is the Sun at that moment vertical ? 

PROBLEM XXIV. 

The day and hour having been given at any place, to find where 
the Sun is rising, setting, where it is noon, those places that have 
the morning twilight, those having evening twilight, and those 
places ivhere it is midnight at that time. 

Rule. — Find the place where the Sun is then vertical, elevate 
the globe for that place, and bring it to the meridian. All those 
places along the western edge of the wooden horizon have the Sun 
rising ; to those along the eastern edge, he is setting ; those under 
the brass meridian, above the horizon, have noon ; those under 
the brass meridian, below the horizon, have midnight ; that par- 
ticular point under the degree of the Sun's declination on the brass 
meridian, has the Sun vertical; all those places situated within 
eighteen degrees below the eastern edge of the horizon, have even- 
ing twilight; and those situated within eighteen degrees below 
the western edge of the horizon, have morning twilight. 

EXAMPLES. 

1. AVhen it is 10 o'clock in the morning at St. Louis on the 5th 
of August, where is the Sun then rising ? 

Ans. At Kamschatka, the Sandwich Islands, and the Georgian 
Islands. 

Where is he then setting ? 

Ans. Siberia, Sea of Aral, southern part of the Caspian Sea, 

Bassora, middle of the Red Sea, and middle of Africa. 
Where is he then vertical ? 
Ans. At the island of Antigua. 
Where is it then noon ? 
Ans. At Baffin's Bay, Labrador, Nova Scotia, Carribee 

Islands, mouths of the Orinoco, middle of South America, 

and Falkland Islands. 
At what places is it then midnight ? 
Ans. Middle of Siberia, eastern part of China, eastern part 

of the islands of Borneo and Java, and the western part 

of New Holland. 
Where is it morning twilight ? 
Ans. Those islands in the Pacific Ocean between the San- 

wich and Miilgrave Islands, and between the Georgian and 

Friendly Islands. 
Where is it evening tAvilight at that time ? 
Ans. Tartary, Persia, eastern part of Arabia, and eastern 

coast of Africa. 



302 bouvier's familiar astronomy. 

2. When it is 2 o'clock in the morning at St. Michael's, one 
of the Azores, on the 27th of July, at what places is the Sun rising 
and setting at that time ? Where is he vertical ? What places 
have noon ? What have midnight ? 

3. At Edinburgh at 6 a. m., where is it noon ? sunrise? sunset? 
midnight ? and where is the Sun vertical ? 

4. At Charleston at noon, where is the Sun vertical ? where is 
it rising ? setting ? and where is it midnight ? 

5. At midnight at Philadelphia, where is the Sun vertical ? 
where is it rising ? setting ? and where is it noon ? 



PROBLEM XXV. 

A place being given in the north frigid zone, to find the length 
of the longest day and night there ; likewise the fij^st and last days 
of his appearance. 

Rule. — Subtract the latitude from 90°, which is the co-latitude, 
and reckon an equal number of degrees on the brass meridian 
from the equator, north and south ; under the points where the 
reckoning ends, bring that point of the ecliptic between Aries and 
Cancer, and observe what point of it passes under the above mark 
on the brass meridian ; this point will give the Sun's place when 
the longest day commences. Turn the globe westward, and ob- 
serve the next point of the ecliptic which comes under the degree 
of the brass meridian thus marked ; this point will give the Sun's 
place when the longest day ends, and the day corresponding to it 
will be the last day on which the Sun will constantly shine with- 
out setting. The number of days between the two will be the 
length of the longest day in the given place. Turn the globe 
westward again, and mark the point of the ecliptic that comes 
under the degree on the brass meridian, situated below the equa- 
tor, which is equal to the co-latitude which was found; this point 
will give the Sun's place corresponding to the last day of his ap- 
pearance above the horizon, or the beginning of the longest night. 
Turn the globe westward again, and observe the next point of the 
ecliptic which falls under the same degree ; this will be the Sun's 
place when the longest night ends. The number of days between 
the end of the longest day and the beginning of the longest night, 
together with the number of days from the end of the longest 
night to the beginning of the longest day, will be those days on 
which the Sun will rise and set alternately every 24 hours. 

By the Analemma, 

The two days on the analemma that pass under the co-latitude 
on the brass meridian north of the equator, will be the beginning 



PROBLEMS ON THE TERRESTRIAL GLOBE. 303 

and end of the longest day; and those two days that pass under 
the co-latitude south of the equator, will be the beginning and 
end of the longest night. 

By Calculation. 
Subtract the latitude from 90°, and the remainder will be the 
co-latitude. The Sun being in the ascending signs, find by the 
table on what day his declination is equal to the co-latitude, but 
of a contrary name ; this will denote the day when the Sun first 
appears above the horizon : find the same when the Sun is in one 
of the descending signs ; this will be the day on which he entirely 
disappears. In the same manner, find the two days w^hen the Sun's 
declination is equal to the co-latitude, and of the same name with it : 
the one will be the beginning, the other the end, of the longest day. 

EXAMPLES. 

1. When does the Sun begin to appear above the horizon at 
the island of Jan Mayen, lat. 71° 10'? when does it disappear? 
how many days are the inhabitants without seeing the Sun ? and 
what is the length of their longest day ? 

Ans. The co-latitude is 18° 50' ; this being counted on the brass 
meridian on both sides of the equator, the four points of the 
ecliptic that pass under it will correspond to May 15, July 28, 
November 14, and January 26. Consequently, the longest day 
begins on the 15th of May and ends on the 28th of July, and is 
therefore equal to 74 days of 24 hours each. The longest night 
commences on the 14th of November and ends on the 26th of 
January, and is therefore 73 days long, during which time the in- 
habitants do not see the Sun. 

2. What is the length of the longest day and longest night on 
the northern part of the island of Spitzbergen, in lat. 80° N. ? 
when do they begin and end ? 

Ans. The longest day begins on the 15th of April, and ends 
on the 27th of August. The longest night commences on the 18th 
of October, and ends on the 22d of February. 

3. What is the length of the longest day and longest night at the 
North and South Poles ? when do they commence ? and what is the 
difference between the summer and winter half year at both poles ? 

4. What is the length of the longest day and longest night at 
Mount Heckla, in Iceland ? 

5. What is the length of the longest day and night at Melville 
Island ? 

PROBLEM XXVL 

To find in what latitude in the frigid zone the Sun begins to 
shine without setting on any given day. 

KuLE. — Find the Sun's declination for the given day by the 



304 



BOUVIER S FAMILIAR ASTEOXOMY. 



table, and subtract it from 90° ; the remainder will be the latitude 
required. For the north frigid zone the given day must be be- 
tween the vernal equinox and summer solstice, or from the 21st 
of March to the 21st of June. For the south frigid zone the 
given day must be between the autumnal equinox and winter sol- 
stice, or between the 23d of September and 21st of December. 



EXAMPLES. 



un begin to shine without setting 
Ans. Lat. 67^ 49^ X. 



1. In what latitude does the 
on the 2d of June ? 

2. In what latitude does the Sun begin to shine without setting 
on the 17th of October? Ans. Lat. 80^ 47^ S. 

In what latitude does the Sun begin to shine without setting on 
the following: davs? — 



3. April 10. 

4. June 21. 
o. May 26. 



6. October 18. 

7. November 20. 

8. December 1. 



PEOBLIM XXVn. 

To find the latitude of those places hi the frigid zone where the 
Sun does not set for a given nurnler of days. 

Rule. — Count as many degrees from the first point of Cancer, 
either way towards the equinoctial point, as is equal to half the 
number of days. Bring this point to the brass meridian, and ob- 
serve the degree cut by it; "subtract this from 90°, and the 
remainder will be the latitude required. 

examples. 

1. In what latitude does the Sun shine continually for 60 days? 

Am. 70° 10'. 

2. In what latitude does he shine continually for 80 days ? 

3. In what latitude does he shine for one month ? 

4. In what latitude does he shine for 2 months ? 3 months ? 4 
months ? 5 months ? 6 months ? 



PROBLEM XXTIII. 

The day and hour at any place being given when a lunar 
eclipse will happen, to find all those places on the globe to which 
it will be visible. 

Rule. — Find the Sun's place for the given time, elevate the 
pole which is most remote from the Sun as many degrees as his 
declination is north or south of the equator. Bring the given 
place to the brass meridian, and set the index to 12 ; then, if the 
given time be before noon, turn the globe westward; but if the 



PROBLEMS ON THE TERRESTRIAL GLOBE. 305 

given time be after noon, turn the globe eastward as many hours 
as the time is before or after noon. The place exactly under the 
Sun's declination will be the antipodes of that place at which the 
Moon will be vertically eclipsed. Keep the globe in this position, 
and set the index to 12 ; turn the globe till the index has passed 
over 12 hours; then all those places above the horizon will have 
the eclipse visible. Those situated on the western edge of the 
horizon will see the Moon rise eclipsed, and those along the east- 
ern edge will see her set eclipsed. Those places situated directly 
under the zenith, or that degree on the brass meridian 90° from 
the horizon, will see her vertically eclipsed.* 

EXAMPLES. 

1. An eclipse of the Moon commenced at Philadelphia on the 
8th of March, 1849, at 6/i. 20m. P. M., and ended at 9A. 20m. P. M. : 
where was it visible ? 

Ans. The Sun's declination being 4° 59' S., the north pole should 
be elevated 4° 59', and Philadelphia be brought to the meridian, 
and the index set to 12. The given time being 9A. 20m. after 
noon, the globe should be turned eastward till the index has 
passed over 9/i. 20m. The index now should be set again to 12, 
and the globe be turned until the index has passed over 12 hours, 
when it will be seen that it rose eclipsed at Chicago, Louisville, 
Cincinnati, Mobile, Nashville, Natchez, and New Orleans, in the 
United States ; the eastern shores of Yucatan, Nicaragua ; and 
at Quito and Truxillo, in South America. It was vertical at 
the coast of Guinea, Liberia, and Sierra Leone, in Africa; and 
rose eclipsed in the middle of Siberia and Tartary, at Ava and 
Umerapoora, in Asia, and on the western shores of the island of 
Sumatra. 

2. On the 20th of April, 1837, at Ih. 40m. p. M., mean time 
at Washington, the Moon was totally eclipsed: where was the 
eclipse visible, it having lasted 3/i. 42m. ? 

Ans. It rose eclipsed at North Cape, Bergen, and Drontheim, 
in Norway ; at London, Rochelle, Gibralter, and Sierra Leone. 
It was vertically eclipsed at a point in the Indian Ocean, in Ion. 
78° E., lat. 10° 39' S. ; and also to all places situated in that lati- 
tude, as far west as 22° E. Ion. It set eclipsed at the Kurile 
Islands, Mulgrave Islands, Friendly Islands, and New Zealand. 
The eclipse was visible throughout almost all Europe, the whole 
of Asia, Africa, and Australia, but Avas invisible in North and 
South America. 

■* The Moon must be vertical to more than one phiee during her eclipse, for the Earth, 
being constantly rotating on her axis, brings now phices in succession untler the Moou 
during the time of her obscuration. 

20 



306 bouvier's familiar astronomy. 

3. On the 27tli of August, the moon was partially eclipsed, the 
beginning of the eclipse being at 3/i. 59???. in the morning, and 
ending at 6/i. 31m. a. m., mean time at Boston : where was it 
visible. 

4. On the 10th of May, 1808, there was a total eclipse of the 
Moon, when it was 8 o'clock in the morning at Greenwich ; where 
was it visible ? 

PROBLEM XXIX. 

To illustrate the phenomenon of the Sarvest Moon. 

Rule. — For North Latitude. Elevate the north pole to the 
latitude of the place, moisten small pieces of paper and stick them 
on the ecliptic, 12° apart, beginning at the point Aries, and ex- 
tending each way, until there are 12 or 13 pieces attached to the 
globe. Then turn the globe westward on its axis until the piece 
of paper nearest to the sign Pisces comes to the eastern part of the 
horizon, and set the index to 12. Turn the globe westward on 
its axis until the other pieces come to the horizon successively ; 
the intervals of time shown by the index between the arrivals 
of the pieces to the horizon will show the difference of time be- 
tween the Moon's rising every day."^ 

For the South Latitude. — Elevate the south pole to the latitude 
of the given place, mark the point Libra, and every 12° preceding 
and following that point. Bring that mark which is nearest to 
Virgo to the eastern edge of the horizon, and set the index to 12. 
Turn the globe westward until the other marks come to the hori- 
zon, and observe the hours passed over by the index, (or the hours 
must be observed on the equator ;) the intervals of time between 
the successive appearance of the marks at the eastern horizon 
will be the difference of time between the Moon's rising. 

The Harvest Moon in south latitudes is the full moon which happens about the vernal 
equinox; for as the seasons are the reverse from those in the northern hemisphere, the 
Harvest Moon in southern latitudes is in Virgo and Libra. 



* The Sun is in Virgo and Libra in the autumnal months ; the Moon is always full 
when she is in the opposite signs, which are Aries and Pisces, which consequently 
occurs in the autumn months. And although the Moon is in these signs twelve times 
in the year, it is only about the autumnal equinox that her orbit is nearly parallel with 
the horizon, so that there is very little difference in her rising for several nights. 

In autumn, the signs Pisces and Aries being opposite to the Sun's place, they rise 
when the Sun sets ; and as the Moon is in them when full, her rising is very remarkable. 



PROBLEMS ON THE CELESTIAL GLOBE. 307 

CHAPTER H. 

PROBLExM I. 

To find the right ascension and declination of a star. 

Rule. — Bring the star to the brass meridian ; the degree of the 
meridian over the star is the declination, and the degree of the 
equinoctial under the meridian is the right ascension. 

Right ascension may be expressed in degrees or hours, and the declination in degrees 
north or south of the equinoctial; or in north polar distance, which is the number of 
degrees from the North Pole, counting from 0^ to 180°, which is the extremity of the 
South Pole. 

In order to find the north polar distance, if the star be north declination, subtract the 
number of degrees which it is north of the equinoctial from 90°, and the remainder will 
be the north polar distance, or, which is the same thing, the distance from the North 
Pole: for example, the star a Tauri, or Aldebaran, is 16° 11' declination. In order to 
find its north polar distance, we subtract 16° 11' from 90°, which leaves 73° 49', the 
north polar distance of Aldebaran. In like manner, if the star be south declination, 
add the number of degrees it is south of the equinoctial to 90°, and the sum will be the 
north polar distance. Thus, Sirius, or a Canis Major, is 16° 30' south declination ; 
therefore, 16° 30', added to 90°, equals 106° 30', which is the north polar distance of 
Sirius, or his distance from the North Pole. 

EXAMPLES. 

1. Required the right ascension in degrees and time, the de- 
clination, and north polar distance of Wega, or a Lyra. 

Ans. R. A.18A. 31m.,or277°45'; Dec. 38° 38' N. ; N. P. D. 
51° 22'. 

2. Required the R. A. in degrees and time, the Dec, and N. 
P. D. of Rigel^ or /9 Orionis. 

Ans. R. A. bh. 7m., or 75° 45' ; Dec. 8° 23' S. ; N. P. D. 
98° 23'. 



3. Algol, or /9 Persei. 

4. Arcturus, or a Bootis. 



5. a Orionis, or Betelgeuse. 

6. Canopus, or a Argus. 



PROBLEM XL 

The right ascension and declination, or right ascension and 
north polar distance, of a star being given, to find it on the globe. 

Rule. — Bring the right ascension marked on the equinoctial to 
the brass meridian, then under the given declination marked on 
the meridian will be the star required. 

EXAMPLES. 

1. What is the name of that star whose right ascension is 20^. 
9m. 10s. in time, or 302° 17' 30" in degrees, and 13° south de- 
clination, or 103° north polar distance ? Ans. a Capricorui. 



308 bouvier's familiar astronomy. 



Required the stars whose rio;ht ascension and declination are 


as follows : 






2. E.A. 


212° 0'. 


Dec. 20° 0' N. 


3. " 


99° 30'! 


" 16° 30' S. 


4. " 


29° 30'. 


" 22° 42' N. 


5. " 


Oh, Om. 


KP.D. 31° 44'. 


6. " 


IbJi. 56m, 


" 109° 21'. 


7. " 


lA. Om. 


" 1° 30'. 

PROBLEM III. 



To find the latitude and longitude of a given star. 

Rule. — Bring that pole of the ecliptic which is in the same 
hemisphere with the given star to the brass meridian, and fix over 
it the quadrant of altitude. Hold the globe steady, move the 
quadrant till it comes over the given star ; then the degree of the 
quadrant cut by the star is its latitude, and the degree on the 
ecliptic cut by the quadrant is its longitude. 

EXAMPLES. 

1. Required the latitude and longitude of Aldeharan^ or a 
Tauri. Ans, Lat. 5° 28' S., Ion. K 6° 53'. 

2. Required the latitude and longitude of Pollux, or /9 Gemini. 

Ans, Lat. 6° 40' N., Ion. @ 20° 28'. 

3. Regulus, or a Leonis. 

4. Rastaben, or y Draconis. 

5. Fomalhaut, or a Pisces Australis. 

6. Algol, or /9 Persei. 

PROBLEM IV. 

The day of the month being given, to find at tvhat hour any 
star comes to the meridian. 

Rule. — Bring the Sun's place to the meridian, and set the 
index to 12 o'clock. Turn the globe round till the given star 
comes to the meridian, and the index will show the hour. 

If the star be east of the Sun, it will not come to the meridian till after the Sun ; con- 
sequently it will be p. M. But if the star be west of the Sun, it will come to the meri- 
dian before him ; and therefore the hour will be A. jr. 

EXAMPLES. 

1. At what hour does Aldebaran come to the meridian on 
February 9 ? Ans, 6h. 55??z. p. M. 

At what hour do the following stars come to the meridian on 
Christmas-day ? — 

2. Sirius, or a Leonis. 

3. Capella, or a Auriga. 

4. Mizar, or ^ Ursa Majoris. 



o 



5. Canopus, or a Argus. 

6. Procyon, or a Canis Minoris. 

7. Caph, or /9 Cassiopeiee. 



PROBLEMS ON THE CELESTIAL GLOBE. 309 

At what hour will the following stars come to the meridian on 
the respective days ? — 

8. Regulus, or a Leonis, April 1. 

9. Seginus, or y Bootis, September 7. 

10. Graffias, or ^ Scorpiij May 3. 

11. Albireo, or ^ Cygni, August 1. 

12. Phset, or a Columba, October 10. 

13. Scheat, or ^ Pegasi, August 31. 

PROBLEM V. 

To find on what day of the year any star passes the meridian 
at any given hour. 

Rule. — Bring the given star to the meridian, and set the index 
to the given hour. Turn the globe till the index points to 12 at 1 J 

noon ; the day of the month corresponding to the degree of the f 1 

ecliptic under the meridian will be the day required. 

EXAMPLES. 

1. On what day does Spica Yirginis come to the meridian at 
half-past nine in the evening? A7is. May 18. 

On what days do the following stars come to the meridian at 
10 in the evening ? — 

2. Algol, or /5 Persei. 

3. Canopus, or a Argus. 

4. Yindemiatrix, or e Virginis. 

5. Fomalhaut, or a Pisces Australis. 

6. Kalpeny, or /9 Aquarii. 

7. Wega, or a Lyrge. 
On what days does Aldebaran come to the meridian at — 

8. Noon? 10. 9 p.m. 

9. 6 A. m. 11. Midnight. 
On what days do the following stars come to the meridian at 

3 A. M. ? 

12. Betelgeuse, or a Orionis. 

13. Sirius, or a Canis Majoris. 

14. Benetnasch, or v^ Ursa Majoris. 

PROBLEM VI. 

The latitude^ hour, and day of the month being given, to f)id 
the altitude and azimuth of any star. 

Rule. — Elevate the globe for the given latitude, bring the 
Sun's place to the meridian, and set the index to 12. Turn the 
globe till the index points to the given hour, screw the quadrant 



310 bouvier's familiar astronomy. 

of altitude on the zenith, and bring it over the star ; then the de- 
gree upon the quadrant cut by the star will be its altitude, and 
the degree of the wooden horizon cut by the quadrant will be the 
azimuth. 

EXAMPLES. 

1. What is the altitude and azimuth of Sirius, or a Canis Ma- 
joris, at Washington, at 9 o'clock in the evening on Christmas- 
day? Ans. Alt. 17°, azimuth 51° E. 

What are the altitude and azimuth of the following stars at 
Philadelphia, May 1, at midnight ? — 



2. Raselgeti, or a Herculis. 

3. Wega, or a Lyrse. 



4. Dubhe, or a Ursa Majoris. 

5. Spica, or o. Virginis. 



What are the altitude and azimuth of the following stars at the 
Cape of Good Hope, June 1, at 9 in the evening ? — 



6. Spica, or a Virginis. 
T. Acrux, or a Crucis. 



8. Antares, or a Scorpii. 

9. Agena, or ^ Centauri. 

Required the altitude and azimuth of the following stars at 
Paris, October 5, at 10 in the evening ? — 

10. Alpheratz, or a Andro- 

meda. 

11. Mira, or o Ceti. 



12. Mesartim, or y Arietis. 

13. Algol, or {i Persei. 



PROBLEM VII. 

The azimuth of a star and the day of the month being given, to 
find the hour and the altitude of a star in a given latitude. 

Rule. — Rectify the globe as in the last problem, screw the 
quadrant of altitude on the zenith, and bring it to the given azi- 
muth. Turn the globe till the star comes to the quadrant ; the 
index will then show the hour, and the altitude of the star will bo 
shown by the quadrant. 

EXAMPLES. 

1. The azimuth of Antares, or a Scorpii, at Philadelphia, on 
the 21st of June, was S. 25° W. : required the altitude and hour 
of the night. Ans. Hour, midnight ; alt. 20°. 

Having found the azimuth of the following stars, required their 
altitude and their hour for London' on September 1. 

2. Ras Alphague, or a Ophiuchi, S. 48° W. 

3. Benetnasch, or Tj Ursa Majoris, N. 51° 30' W. 

4. Alphecca, or a Coronse Borealis, S. 1° W. 

5. Arcturus, or a Bootis, N. 81° W. 

6. Mirach, or ^ Andromeda, N. 68° E. 



PROBLEMS ON THE CELESTIAL GLOBE. 311 

PROBLEM VIII. 

The altitude, of a star, the day and the latitude being given, to 
find the hour and the azimuth. 

Rule. — Rectifj^ the globe as in the preceding problems, screw 
the quadrant of altitude upon the zenith, then turn the globe and 
move the quadrant till the star cuts the quadrant at the given alti- 
tude ; the index will show the hour, and the quadrant the azimuth 
on the horizon. 

As all the stars have the same altitude twice a day, it is necessary to state whether 
the star is east or west of the meridian. 

EXAMPLES. 

1. The altitude of Alderamin, (a Cephei,) at Alexandria, in 
Egypt, on the morning of August 9, was 41° : required the hour 
and the azimuth. Ans. Hour, 4 A. M. ; azimuth, N. 34° W. 

Having the altitudes of the following stars at New York, on the 
days given, required the time and the azimuth : 

2. February, 15 p. M., Procyon, or a Canis Minoris ; alt. 48° 30'. 

3. December 25, P. m., Aldebaran, or a Tauri ; alt. 30°. 

4. July 4, A. M., Aldebaran ; alt. 14° 30'. 

Having the altitudes of the following stars at Quito, required 
the hour and azimuth: 

5. May 12, A. M., Fomalhaut, or a Pisces Australis ; alt. 34°. 

6. June 30, p. M., Wega, or a Lyrse ; alt. 24°. 

7. December 19, A. M., Acrux, or a Crucis ; alt. 21°. 

8. March 3, A. M., Antares, or a Scorpii ; alt. 36°. 

9. January 1, p. M., Phset, or a Columbse ; alt. 26° 30'. 

10. October 10, p. M., Alpheratz, or a Andromedse ; alt. 62°. 

11. September 5, A. M., Menkar, or a Ceti ; alt. 70° 40'. 

PROBLEM IX. 

Having the azimuth of a star, the latitude and tlie hour, to find 
the stars altitude and the day of the month. 

Rule. — Elevate the globe for the latitude of the place, screw 
the quadrant of altitude on the zenith, and bring it to the given 
azimuth; bring the star to the edge of the quadrant, and set the 
index to the given hour, and the degree of the quadrant cut by 
the star will be its altitude. Turn the globe till the index points 
to 12, and the day of the month marked on the ecliptic where it 
is cut by the brass meridian, is the day required. 

EXAMPLES. 

1. The azimuth of Arcturus at 6 p. M., at Philadelphia, was ob- 
served to be N. 80° W. : what was its altitude, and the day and 
month ? Ans. Alt. 21° ; October 15. 



312 bouvier's familiar astronomy. 

2. At Washington City, at 9 o'clock in the evening, the azi- 
muth of Aldebaran was found to be S. 89° W. : required its alti- 
tude, and the day and month. Ans. Alt. 26° ; March 21. 

3. At the Cape of Good Hope, at midnight, the azimuth of 
Sirius was S. 83° W. : what was its altitude, and the day and 
month ? 

4. At Quito, at 10 o'clock, the azimuth of Achernar was S. 30° 
W. : required its altitude, and the day and month. 

5. At Philadelphia, at 5 o'clock in the morning, the azimuth 
of a Arietis was S. 79° W. : what was its altitude, and the day and 
month ? 

6. At Eome, at 5 o'clock in the morning, the azimuth of Ca- 
pella was observed to be N. 65° W. : required its altitude, and the 
day and month. 

PROBLEM X. 

The latitude and day of the month being given, to find the hour 
of the night hy observing when tivo stars have the same azimuth. 

Rule. — Elevate the pole to the given latitude, screw the quad- 
rant of altitude on the zenith, bring the Sun's place in the ecliptic 
to the brass meridian, and set the index to 12 ; move the globe 
and the quadrant, till the quadrant comes over both stars ; the 
hour will be shown by the index. 

EXAMPLES. 

1. At New York, on the 8th of February, the stars Betelgeuse 
and Rigel were observed to have the same azimuth : what was 
the hour ? Ans. 10 o'clock, p. m. 

2. At Jerusalem, on the 3d of September, the stars Almaae 
and Capella were observed to have the same azimuth : what was 
the hour? Ans. 2 o'clock a.m. 

Required the hour at Nova Zembla, when the following stars 
have the same azimuth on the annexed days: 

3. April 11, Etanin and Vega. 

4. September 10, Capella and Betelgeuse. 

5. March 31, Cor Caroli and Spica. 

6. May 16, Arided and Enif. 

PROBLEM XI. 

To find the time of the rising, setting, and culminating of any 
star, for any given day and place. 

Rule. — Elevate the pole for the given latitude, bring the Sun's 
place to the meridian, and set the index to 12. Turn the globe 
till the star comes to the eastern horizon, and the hour shown by 



PROBLEMS ON THE CELESTIAL GLOBE. 313 

the index is the hour the star rises at that place on that day. 
Bring the star to the brass meridian, and the index will show the 
hour it culminates. Continue to turn the globe, and the moment 
the star comes to the western horizon is the time of its setting, as 
shown bj the index. 

EXAMPLES. 

Required the time when the following stars rise, culminate, and 
set, at the respective places and days : 

1. Spica, at Philadelphia, May 12. 

Ans. Rises at 4h. 40m. p. m. ; culminates at lOA. Om. P. M. ; 
sets at 3/i. 25m. A. M. 

2. Arcturus, at New Orleans, September 21. 

Ans. Rises at Ih. 25m. a. m. ; culminates at 2h. 15m. p. M. ; 
sets at lOh. 10m. p. m. 

3. Menkar, at Moscow, January 1. 

4. Procyon, at Melville Island, June 21. 

5. Alioth, at Demarara, December 19. 

6. Achernar, at Potosi, July 4. 

The degree of the equinoctial that rises with a star is its oblique 
ascension ; the degree of the equinoctial that sets with a star is 
its oblique descension : the distance of a star from the east point 
of the horizon at its rising, is its eastern amplitude ; the distance 
of a star from the west point of the horizon at its setting, is its 
western amplitude. 

Required the oblique ascension, oblique descension, and eastern 
and western amplitude of the following stars, at the respective 
places and days : 

7. Sirius, at London, on the 15th of March. 

Ans. Oblique ascension, 120° 50'; oblique descension, 77° 15'; 
amplitude, 27° S. 

8. Arcturus, at Queen Charlotte's Islands, March 10. 

9. Betelgeuse, at Washington, October 1. 
10. Aldebaran, at Cincinnati, September 30. 

PROBLEM XIL 

To slioiu the appearance of the heavens for any given day and 
hour at any given place. 

Rule. — Elevate the globe to the latitude of the place, bring 
the Sun's place to the meridian, set the index to 12, and turn the 
globe till the index points to the given hour. The constelhitions 
will be represented in their relative situations as they are in the 
heavens. By placing the end of a pencil on any star, the point of 
the pencil will indicate the direction of the same star in the heavens. 



314 bouvier's familiar astronomy. 

EXAMPLES. 

1. Required the appearance of the heavens at Boston, on the 
1st of January, at 9 in the evening. 

Ans. Regulus and Cor Caroli are near the eastern horizon; 
Aldebaran is a little to the east of the meridian ; Sirius and Pro- 
cyon appear in the south-east ; the constellation Orion is about 
two hours to the east of the meridian ; Gemini is in the south-east ; 
Ursa Major is in the north-east ; Wega is just setting ; Andromeda 
and Pegasus are west of the meridian ; and Cetus and Aries are in 
the south-west. 

2. Required the appearance of the heavens at Quito, Septem- 
ber 20, at midnight. 

3. What appearance do the heavens present at Cape Horn, at 
4 o'clock a. m., May 31? 

4. Describe the configuration of the heavens at Cairo, on the 
10th of August, at 6 o'clock, p. m. 

PROBLEM XIII. 

To find what stars never rise or set at any place. 
Rule. — Elevate the pole for the latitude of the place, and then 
hold a pencil at the north point of the horizon, and turn the globe 
round. The circle thus drawn will include, between it and the 
pole, all the stars which never set. Then hold the pencil at the 
south point of the horizon, and turn the globe ; the stars included 
between the circumference of this circle and the depressed pole, 
never rise to that latitude. 

By Calculation. 
Find the co-latitude by subtracting the latitude of the place from 
90°. If the declination of the star is greater than the co-latitude, 
and of the same name with it, it will never set ; if it be greater 
than the co-latitude, and of a contrary name, it will never rise to 
that place. 

examples. 

1. What stars never rise to those places in the same latitude as 
Boston? What stars never set to places in that latitude ? 

Ans. Those constellations which never rise are the greater part 
of Argo Navis, Equuleus Pictorius, Dorado, Reticulus, Horologium, 
Solarium, Hydrus, part of Phoenix, Toucana, part of Grus, Pavo, 
part of Telescopium, Octans, part of Norma, Circinus, Apus, Tri- 
angulum, part of Lupus, part of Centaurus, Crux, Musca Aus- 
tralis. Chameleon, Mons Mensse, Pisces Volans, and Robur Caroli. 
Those stars which never sink below the horizon in that latitude 
are the constellation Cassiopeia, part of Perseus, Tarandus, Ca- 
melopardalus, part of Auriga, part of Lynx, the greater part of 



PROBLEMS ON THE CELESTIAL GLOBE. 315 

Ursa Major, including the "Dipper," Quadrans Muralis, Draco, 
Cepheus, and Ursa Minor. 

2. What stars never set at Calcutta? 

A71S. All those situated between the Arctic Circle and the 
North Pole. 

3. What stars never rise at Rio Janeiro ? 

Ans. Those which never set at Calcutta. 

4. How far must a person travel south to lose sight of Capella? 

5. How far south must a person travel to see a Centauri ? 

6. In what latitude is the star Sirius always below the horizon? 

7. In what latitude is Sirius always above the horizon ? 

8. How far south do those live who never see Lyra ? 

PROBLEM XIV. 

TJie latitude of the place being given, and the dag of the month, 
to find what planets will be above the hojHzon after sunset. 

Rule. — Elevate the pole for the latitude of the place, find the 
Sun's place in the ecliptic, and bring it to 10° or 12° below the 
western part of the horizon ; then find the right ascension and 
declination of the planets for the given day in a good ephemeris 
or the Nautical Almanac, and those whose situations are above 
the horizon when the Sun is 10° or 12° below it in the west, will 
be visible either by the naked eye or a good telescope. 

EXAMPLES. 

1. What planets were visible at Philadelphia on the 1st of Sep- 
tember, 1855, after sunset ? 

Ans. The right ascensions and declinations of the planets were, 
at that time, as follows ; 



Mercury, 


KIGHT ASCENSION. 

llh. 2m. 


DECLINATION. 

7° 42' N. 


Venus, 


12h. 42m. 


10° 46' S. 


Mars, 


Sh. 10m. 


21° 4'N. 


Jupiter, 


21h. 5^m. 


13° 58' S. 


Saturn, 


bh. 53m. 


22° 14' N. 


Uranus, 


Sh. Urn. 


17° 39' N. 


Neptune, 


22h. 50m. 


12° 0' S. 



Therefore, at sunset Venus was on the verge of the western hori- 
zon, Jupiter and Neptune were in the eastern horizon ; the rest 
were at that hour invisible. 

2. What planets were visible at Philadelphia on the 1st of Sep- 
tember, 1855, at 4 o'clock, a. m. ? 

A71S. Jupiter and Neptune Avere setting ; Mars had risen, and 
was an hour and a half high ; Saturn was about midway between 



316 bouvier's familiar astronomy. 

tlie eastern horizon and the zenith ; and Uranus was half an hour 
east of the meridian. 

3. What stars will be visible at Boston, at midnight, on the 
first of next month of this year ?^ 

4. What planets will be visible at Cairo, December 1, next, at 
7 o'clock in the evening ? 

5. "What planets will be visible at that place at midnight on 
that day ? 

PROBLEM XV. 

To find on what day and hour a given star culminates. 

Rule. — Bring the given star to the brass meridian, and set 
the index to 12 ; turn the globe westward till the index has 
passed over as many hours as the time is before noon ; but if 
the given time be past noon, turn the globe eastward till the 
index has passed over as many hours as the time is past noon. 
Observe the degree of the ecliptic which is intersected by the 
graduated edge of the brass meridian, and the day of the month 
answering thereto wilLbe the day required. 

examples. 

1. On what day of what month does Spica come to the meri- 
dian at 10 minutes past 10 in the evening ? Ans. May 10. 

2. On what day of what month does Aldebaran culminate at 
midnight ? Ans. December 1. 

3. On what day of what month does Altair come to the meri- 
dian at 11 o'clock, p. M. ? 

4. On what day of what month does Benetnasch culminate 
at 2, A. M. ? 

5. On what day does Cor Caroli culminate at noon ? 



PROBLEM XYI. 

To find the Sun's right ascension, oblique ascension, ascen- 
sio7ial difference, eastern amplitude, and time of rising, at any 
given place on a given day. 

Rule. — Elevate the pole for the latitude of the place, and 
bring the Sun's place to the meridian ; the degree of the equi- 
noctial cut by the graduated edge of the brass meridian is the 
Sun's right ascension. Set the index to 12, bring the Sun's 
place to the eastern edge of the horizon, and the degree of the 
equinoctial cut by the horizon is the Sun's oblique ascension. 
The difference between the right and oblique ascension is the 

* The student must consult an ephemeris for the current year, in order to be enabled 
to solve this and the following question. 



PROBLEMS ON THE CELESTIAL GLOBE. 317 

ascensional difference. The number of degrees on the horizon, 
between the Sun's place and the east point of the horizon, is the 
rising or eastern amplitude. The time shown by the index, when 
the Sun is in the horizon, is the time of his rising. 

EXAMPLES. 

1. Required the Sun's right ascension, oblique ascension, ascen- 
sional difference, eastern amplitude, and time of rising at Quebec, 
on the 5th of November. 

Ans. Right ascension, 14A. 42m. ; oblique ascension, 240° ; 
ascensional difference, 20°; eastern amplitude, 23° 20'; Sun 
rises, 7 A. 10 m. 

2. Required the Sun's right ascension, oblique ascension, as- 
censional difference, eastern amplitude, and time of rising, at St. 
Petersburg, on the 20th of May. 

3. The same for Valparaiso, on the 1st of January. 

4. The same for the Cape of Good Hope, on the 4th of July. 

5. The same for the island of Juan Fernandez, on the 6th of 
October. 

6. The same for North Cape, on the 15th of June. 

PROBLEM XVII. 

To find the Sun's oblique deseension, descensional difference, 
western amplitude, and time of setting, for any place, on a given 
day. 

Rule. — Proceed as in the last problem, only bring the Sun's 
place to the western horizon. 

EXAMPLES. 

1. Required the Sun's oblique descension, descensional differ- 
ence, western amplitude, and time of setting, for the Cape of 
Good Hope, on the 19th of July. 

Ans. Oblique descension, 103° 44' ; descensional difference, 
15°; western amplitude, 25° 30'; time of setting, 5/i. 

2. The same for Calcutta, on the 21st of June. 

3. The same for San Francisco, on the 1st of January. 

4. The same for Truxillo, on the 8th of May. 

5. The same for Boston, on the 5th of February. 

6. The same for New Orleans, on the 12th of November. 

PROBLEM XVIII. 

The latitude, day of the month, and hour being given, to find 
the Suns altitude and azimuth. 

Rule. — Elevate the globe for the given latitude, bring the 
Sun's place to the meridian, and set the index to 12 ; then turn 



318 bouvier's familiar astronomy. 

the globe till the index points to the given hour. Screw the 
quadrant of altitude on the zenith, and bring it over the Sun's 
place ; the degree of the quadrant cut by the point of the ecliptic 
in which the Sun is situated, will be his altitude ; and the dis- 
tance between the graduated edge of the quadrant and the north 
and south points of the horizon, will be his azimuth. 

EXAMPLES. 

Kequired the Sun's altitude and azimuth at the following places 
and times : 

1. Washington, May 18, 1h. 30???. A. M. 

Ans. Altitude, 30° ; azimuth, N. 87° E. 

2. St. Petersburg, December 21, llli. 20m. a. m. 

Ans. Altitude, 6° ; azimuth, S. 11° E. 

3. Lima, August 31, 6h. SOm. P. M. 

4. Bombay, April 1, Sh. 4:0m. a. m. 

5. Lake Tchad, June 21, 7^. Om. P. M. 

6. Gottingen, March 10, 4/i. Om. P. M. 

PROBLEM XIX. 

To find the time of the Moons southing, at any given i)lace, 
at any given day. 

Rule. — Find the Moon's place — that is, her right ascension 
and declination, or her longitude and latitude — by a good ephe- 
meris or a nautical almanac, and mark her place on the globe 
by a little patch of paper moistened and stuck on it. Bring the 
Sun's place to the brass meridian, and set the index to 12 ; turn 
the globe westward until the Moon's place comes to the brass 
meridian, and the hours passed over by the index will show the 
hours from noon when the Moon will be on the meridian. 

examples. 

1. On the 1st of November, 1855, the Moon's right ascension 
was 8A. 39w., and her declination 23° 52' N., at noon : at what 
hour on that day did she pass the meridian ? 

Ans. The Moon's time of southing was 18A. 30m. afternoon, or 
6A. 307?z. in the morning. 

2. On the 1st of January, 1855, the Moon's right ascension 
was Dh. 16m., and her declination was 25° 34' X., at noon : at 
what hour did she pass the meridian on that day ? 

3. On the fifth of February, 1810, the Moon's R. A. was 10^. 
44??z. 36s., and her Dec. 4° 48' N. : what time did she pass the 
meridian ? 

4. On the 31st of December, 1811, the Moon's R. A. was 8^. 
2m. IQs., and her Dec. 17° 7' N. : when did she pass the meridian ? 



PROBLEMS ON THE CELESTIAL GLOBE. 319 

PROBLEM XX. 

To find when an eclipse of the Sun or Moon is likely to occur 
in any year. 

Rule. — By the Nautical Almanac or a good ephemeris, find 
the places of the Moon's nodes for the given time ; also, find the 
Sun's place for the given day; then find the Moon's place by the 
Nautical Almanac. If, at the time of full moon, (which happens 
when the Sun and Moon are directly opposite to each other, or 
180° apart,) the Sun should be within 12° of the Moon's node, 
there will be an eclipse of the Moon. But if the Sun and Moon 
have nearly the same right ascension and declination, (which 
always happens at the time of new moon,) or if, at the time of new 
moon, the Sun be within 17° of the Moon's node, there will be an 
eclipse of the Sun. 

EXAMPLES. 

1. In 1855, on the 15th of May, the Sun's and Moon's right 
ascension was 3/i. 29/7z. ; the longitude of the Moon's node was Is. 
12° 9'; Moon's declination, 20° 5' N. ; and Sun's declination, 18° 
bQ' N. : was there an eclipse ? and if so, was it of the Sun or Moon ? 

Ans. As the longitude of the Moon's node was Is. 12° 9' or 
42° 22', and the Sun's R. A. 2>h. 29m. or 52° 15', it follows that 
the Sun will be only 9° 53' from the Moon's node ; and as the 
Sun and Moon have the same right ascension, it must be new 
moon; therefore, there was an eclipse of the Sun. 

2. On the 3d of January, 1855, there was full moon ; the longi- 
tude of the Moon's node was 49° 28' ; the Sun's right ascension 
187z. 54m. ; his declination 22° 51' S. ; the Moon's R. A. was Ih. 
4m., and her Dec. 26° 44' N. : was there an eclipse of the Moon 
on that day ? 

Ans. There was not ; for the Sun being more than 12° from 
the Moon's node, there could not be any eclipse. 

3. There was full moon on the 26th of February, 1812, at 
which time the Moon's node was 158° 9', and the Sun's longitude 
337° 33' ; was there an eclipse of the Moon at that time ? 

4. Was there an eclipse of the Sun on the. 9th of November, 
1855, when the right ascension of the Sun and Moon was 147i. 
57m., the longitude of the Moon's ascending node 32° 56', the 
Dec. of the Moon 18° 8' S., the Sun's Dec. 16° 47' N. ? and if 
so, was it an eclipse of the Sun or of the Moon ? 

5. On the 27th of March, 1812, there was full moon, at which 
time the place of the Moon's node was 5s. 6° 35', and the Sun's 
longitude 7° 11' 14": was there an eclipse at that time? and if 
BO, was it of the Sun or Moon ? 



pistorg of ^stroiiomg. 

CHAPTER I. 

SECTION I. 

cl^rom tlje %nxlmt ^imzs to flje Cljristian €ra. 

"The only rational and just method of writing the history of a science, is to base it exclusively 
on works, the date of whose publication is certain. All beyond this must be confused and 
obscure." — Akago. 

Astronomy is one of the most ancient of the physical sciences, having had its 
devotees from the earliest times to the present day. The greater part of its his- 
tory having been lost in the lapse of ages, no certain conclusions can be gathered 
from the few shreds of truth which have come down to us, interwoven as they are 
with the falsehood and superstition of various nations. Yet, through the sagacity 
of those learned men who have investigated this subject, we are in possession of 
numerous facts which serve as landmarks whereby to direct our course through 
this labyi'inth. Although we may meet with much error, there are many unde- 
niable truths which merit the attention of the student, and serve to guide him in 
forming conclusions which are, after all, only an approximation to the truth ; for, 
having no certain dates on which to rely, the early history of this science must 
necessarily be obscure. 

The heavenly bodies first claimed the attention of man on account of an influ- 
ence they were supposed to exert over the destinies of his race. A knowledge 
of the configurations of the heavens, and the art of predicting future events by 
the appearances of the stars, constituted a science called Astrology, which formerly 
engrossed the attention of the most learned. 

It is to Astronomy we are indebted for our only accurate measures of time. The 
first period noted by man was no doubt counted by days or suns, then by months or 
moons, after which the apparent annual revolution of the Sun gave the idea of a year. 

The various phases of the Moon and planets were supposed to foreshadow the 
events of human life ; for which reason the appearances and movements of those 
bodies were anxiously watched, and often carefully recorded. 

The Sun, possessing a benign influence on all animal and vegetable life, and 
being the fountain of light and heat, became an object of worship at a very early 
period. 

ASTRONOMY OF THE INDIANS. 

Our first accounts of the science of Astronomy are supposed to be derived from 

India : though the fact has never been -proved beyond all doubt, still there is an 

accumulation of evidence in favor of its high antiquity and Indian origin. The 

Astronomy of India is considered more remote in its date than that of any other 

320 



HISTORY OF ASTRONOMY. 321 

nation. It undoubtedly presents a curious problem for the investigation of the 
learned, and one which is yet involved in great uncertainty. 

It is undeniable that the Indian nations have more solid foundations than any 
others on which to rest their claims to precedency in this noble science. But 
whence did their knowledge originate? Was it communicated to them by a 
former race of men, of whom we are ignorant ? We are in possession of Indian 
astronomical tables, and methods of computing the eclipses and places of the 
planets, forming altogether a treatise on Astronomy. Bailey and Professor Play- 
fair imagined they could discern, in the computations and astronomical tables of 
the Indian nations, proofs of their having been made by a race who had derived 
their knowledge from some more ancient people. 

There are several sets of Indian astronomical tables. The first ever known in 
Europe was introduced by La Loubere, an ambassador to India from the court 
of Louis XIV. A set of tables was brought from the Carnatic by Father du 
Champ, in 1750. Another was introduced about the same time by Patonillet, 
which is supposed to have come originally from Narsapour. A fourth set was 
brought by Le Gentil, a French astronomer, who had gone to India in order to 
witness the transit of Venus in 1769. These tables were communicated to Le 
Gentil by a learned Brahmin, and bear a date of much higher antiquity than any 
of the others. The epoch assumed by them is about the year 1300 b. c, and ac- 
cording to which the tropical year consists of but one minute, five seconds, and 
five-tenths more than the year according to Lacaille. 

But this high antiquity claimed by the Brahmins is regarded by many of the 
learned as unworthy of credit ; their calculations are suspected of having been 
extended back by means of more modern tables. La Place places the epoch of 
these tables at about the time of Ptolemy, or about a. d. 125, which date he 
proves to be nearly correct from the acceleration of the Moon. 

Still, the phenomena are noted with so much accuracy as to leave little room 
to doubt that the observations were made by a people far advanced in the science. 
The only data, however, from which a rational conjecture can be formed relative 
to the antiquity of this science, are to be found in the existing monuments. 

The Indian zodiac, which strongly resembles our own, is supposed by some to 
have been borrowed from the Greeks or Arabs ; but Sir William Jones thought 
that it was probably invented by the progenitors of the Hindu race. 

Admii'al Smyth, of the Royal Navy, says — " The Hindu claim to antiquity 
stands on higher ground as a curious, but involved, historical problem. I can- 
not," he continues, "but be somewhat influenced by the learning and sagacity of 
Sir William Jones ; the science and judgment of my friend, Mr. T. C. Colebrooke. 
late president of the Astronomical Society ; the persevering spirit of inquiry of Mr 
Davis; and the talents of Schlegel." He further adds — "On the whole, we i^ust 
allow that the early Hindus applied themselves intensely to the pure sciences, since 
they could compute the mean motions and true places of the planets, and calculate 
lunar and solar eclipses ; they understood the astronomical sphere and its circles; 
suspected a libration of the equinoctial points ; had a glimpse of geometry ; were 
expert in instrumental observations ; and enriched science with those powerful 
organs — arithmetic and algebra " 

21 



322 bouvier's familiar astronomy. 

ASTRONOMY OF THE CHINESE. 

The Chinese also claim the honor of having cultivated the science of Astronomy 
from a very early period ; but, like the Hindus, their pretensions are disputed by 
some and sustained by others among the learned. The truth certainly rests in 
great obscurity, but there are facts which serve to prove an acquaintance with the 
heavenly bodies, as well as a certain degree of advancement in the methods of ob- 
servation, the accuracy of which would imply some knowledge of instruments. 

About 1100 years before the Christian era, certain astronomical observations 
were made in the city of Layang, in China, at the time of the summer and winter 
solstices. The obliquity of the ecliptic thus determined was 23° 54^ 3-15^^. This 
determination of the position of the winter solstice corresponds with the calcula- 
tions of La Place to within V of arc. La Place considers this astonishing con- 
formity as a certain proof of the authenticity of those observations. 

The Chinese boast of a series of eclipses extending over a period of more than 
3000 years, which, however, are not to be relied on, as they are supposed to have 
been computed back. They are only mentioned in the works of Confucius, the 
great Chinese philosopher, as mere facts, and are unaccompanied by any observa- 
tions noted at the time. 

The decimal divisions of time, of degrees of the circle, and of weights and 
measures, were in use among the Chinese 4000 years ago. 

ASTRONOMY OF THE EGYPTIANS. 

The Egyptians are generally supposed to have derived all their astronomical 
knowledge from the Oriental nations. They fixed the length of the year to 365|^ 
days, by observing the heliacal rising of the star Sirius, to which they gave the 
name of Thoth, [watch-dog,) because it immediately preceded the overflow of the 
Nile. This period of 365J days was their sacred year ; their civil year consisted 
of only 365 days. 

The diflference between the lengths of the sacred and civil year led the Egyptians 
to the discovery of the sothic or canicular period of 1460 solar years, correspond- 
ing to 1461 civil years of 365 days, which brings back the months and festivals to 
the same seasons. 

That the Egyptians were acquainted with some method of finding the true meri- 
dian line, seems to be proved by the fact that the pyramids are generally found to 
stand with their faces towards the four cardinal points. The builders of those 
monuments must also have been acquainted with the polar point of the heavens. 
It is an ascertained fact that many of the pyramids have narrow entrances, in- 
clined at an angle equal to the altitude of what was then the pole star at its lower 
cul»ination. 

Diodorus Siculus, who lived in the first century before Christ, informs us that 
the ancient Egyptians understood the phenomena of the stations and retrograde 
motions of the planets. Macrobius, a Latin historian, who flourished about the 
fifth century of the Christian era, ascribes to the Egyptians a knowledge of the 
real motions of Mercury and Venus, and says that they regarded these planets as 
satellites of the Sun. 



HISTORY OF ASTRONOMY. 323 

Astronomy is also a useful aid in discovering the dates of ancient monuments. 
As an instance, on the ceiling of a portico among the ruins of Tentyris may be 
seen the twelve signs of the zodiac, placed according to the apparent motion of the 
Sun. According to this zodiac, the summer solstice is in Leo ; from which it is 
easy to compute by the precession of the equinoxes of 50'''' -1 annually, that the 
zodiac of Tentyris must have been made 4000 years ago. Mrs. Somerville relates 
that on one occasion she witnessed an instance of the successful application of As- 
tronomy in ascertaining the date of a papyrus sent from Egypt. The manuscript 
was found in a mummy-case, and proved to be a horoscope of the time of Ptolemy. 
Its antiquity was determined by the configuration of the heavens at the time of its 
construction. 

That portion or division of time consisting of seven days, and commonly known 
by the name of a week, has been used from the remotest times among the Indian 
nations, the Chinese, and Egyptians. It was also known to the Druids of Gaul 
and Britain, which seems an argument in favor of the common origin of the rudi- 
ments of this science. The division of a week was probably suggested by the 
phases of the Moon. Dion Cassius, a Greek historian, who lived about a. d. 250, 
says that the Egyptians were the first people who dedicated each day of the week 
to the Sun or one of the planets. 



ASTRONOMY OP THE CHALDEANS. 

The level country of Chaldea, with its fine, clear atmosphere, is peculiarly 
adapted to astronomical observations, which, according to some authors, extended 
over a period of more than 1900 years. 

As the Chaldeans were shepherds, they would naturally be led to the contem- 
plation of the heavens when engaged in tending their flocks ; and as they were 
noted for their knowledge of the sister science, Astrology, they must have recorded 
the various changes continually occurring in the celestial bodies. Sir William 
Jones was of opinion that the practice of observing the stars originated with the 
rudiments of civil society, and in the country of the Chaldeans. 

They were able to predict eclipses of the Moon by means of a cycle of 223 
months, or about 18 years, at the end of which time the eclipses begin to return 
at the same intervals and in the same order. Delambre, who wrote a history of 
Astronomy, supposes that they carefully recorded all the eclipses which happened ; 
and then, by inspecting their registers, they found the eclipses of the Moon to 
recur after a period of about 18 years. This period they called Saros, said to be 
a Chaldean word, signifying restitution or return. The Chaldeans had other 
cycles founded on the observations of the celestial bodies, which they termed 
Sessos and Neros, the meaning of which is now lost. 

The following memoir appeared in the London Athenaeum for January, 1855. 
It was sent by Colonel Rawlinson to the British Museum, the authorities of which 
communicated it to the Asiatic Society : — 

Birs-i-Nimr(id is an immense shapeless mound, nearly 300 feet high, and from 
200 to 400 feet in width where it reaches the plain. The whole mound is made 
up of crumbling rubbish, except the summit, which stands out like the fragment 



324 bouvier's familiar astronomy. 

of a watch-tower. It has, from time to time, excited the attention of tourists, 
several sketches of it having been drawn and afterwards published. The account 
given by Colonel Rawlinson is very graphic. 

Having gained much experience in the mode of discovering ruins, he immedi- 
ately ordered a perpendicular shaft to be sunk, at a designated point, until it 
should reach a wall. In the course of two months the workmen found the wall, 
which turned off at right angles at each end. Near the corner of this wall a 
small cavity was discovered, which contained a cylinder covered with inscriptions, 
which had remained during twenty-four centuries in its hiding-place. 

The inscriptions were as fresh as when deposited there by the hands, probably, 
of Nebuchadnezzar himself ! Another cylinder, a duplicate of the first, was found 
in another angle of the wall. 

The inscriptions, like the deposits in the corner stones at the present day, give 
the name of the principal ruler, Nebuchadnezzar ; and proceed with a summary 
of the buildings of Babylon, which the king had repaired or erected. It then 
mentions that the " Temple of the Planets of the Seven Spheres," which had been 
built by an early king 504 years previously, or about 1100 b. c, having become 
ruinous, owing to a neglect of the drainage, the god Merodach had put it in 
his heart to restore it ; that he did not rebuild the platform, which was unin\- 
paired, but that all the rest was restored by his commands. 

ASTRONOMY OF THE PH(ENICIANS. 
The Phoenicians probably derived their astronomical knowledge from the Chal- 
deans and Egyptians. As they were the greatest navigators of ancient times, it 
is to be supposed they must have made some observations of the heavenly bodies, 
in order to perform even their short voyages to the islands of the Mediterranean, 
and the coasts of Spain and Africa. 

ASTRONOMY OF THE GREEKS. 

We are indebted to the Greeks for a more perfect knowledge of Astronomy than 
is to be derived from the writings of any other nation ; for the works of many of 
their poets and philosophers have come down to us entire. Hesiod, a Greek poet, 
designates the various seasons by the ^^ turning of the Sun." Thus, he says — 
" Fifty days after the turning of the Sun is the favorable time to begin a voyage." 
Hence, we may infer that the ancient Greeks divided their year by the recTirrence 
of the solstices. 

The motions of the Moon, as well as the apparent motions of the Sun, were 
known to the Egyptians, Indians, Chaldeans, Phoenicians, and most probably to 
the Chinese, before they were observed by the Greeks. Thales, a Phoenician by 
birth, but a citizen of Miletus, is considered as the first person who propagated 
any truly scientific knowledge of Astronomy. It is supposed that he acquired 
some of his learning in his own country and in Crete. He was taught Geometry, 
Astronomy, and Philosophy, under the priests of Memphis. 

Anaximander, an Ionian philosopher, and disciple and successor of Thales, lived 
about 500 B. c. He is said to have pronounced the Earth a sphere, and to have 



HISTORY OF ASTRONOMY. 325 

had some idea of its revolution on its axis. He is supposed to have observed the 
obliquity of the ecliptic. He maintained that in the universe there is an infinity 
of worlds, and that the Moon shines by light of the Sun reflected from her surface. 

Pythagoras, who flouished about 500 b. c, is said to have discovered the morn- 
ing and evening star to be one and the same body. He taught that the Sun is 
fixed in the centre of the system, and that the Earth, with the other planets, re- 
volves round the Sun ; that the planets are inhabited ; that the comets are mate- 
rial bodies, which revolve round the Sun at stated periods ; that the fixed stars 
are suns, shedding light and heat around them. Pythagoras made a geometrical 
discovery with regard to the properties of the right-angled triangle, showing that 
the science of Geometry also engaged a portion of his attention. 

The system of the universe, as taught by Pythagoras, although it has since 
been found to be a very near approximation to the truth, was for a time super- 
seded by the theories of later philosophers. 

The followers of Pythagoras conceived a comet to be a planetary body, which 
reappears after a certain interval, and which approaches as near the Sun as the 
planet Mercury when at the vertex of the curve it describes. Thus, it appears the 
Pythagorean philosophy, in some respects, strongly coincides with that taught at 
the present day. 

Anaxagoras, who was a cotemporary of Pythagoras, taught that the heavens 
consisted of a solid vault of stones, elevated above the Earth by the surrovmding 
ether, and that the Sun is a huge fiery stone, about the size of the Morea, a 
peninsula of Greece. For this theory he suffered banishment, it being considered 
impious thus to rob the Sun of his divinity, he being believed to be Apollo, one 
of the most powerful deities. According to Cicero, Anaxagoras was the first who 
taught the existence of a Supreme Intelligence, the Creator of all things. 

About 430 B. c, Meton, an Athenian mathematician, constructed the luni-solar 
cycle, consisting of 6940 days, or about 19 years, now called the Metonic CycU. 
in honor of its discoverer. It was inscribed in letters of gold on a marble pillar, 
for which reason the number indicating the order of the current year in that cycle 
is still called the Golden Number. The Metonic cycle was adopted on the 16th of 
July, B. c. 433. 

Democritus, a celebrated philosopher, lived about 400 b. c. He travelled into 
Egypt, Chaldea, and India, by which means he spent a large fortune, and returned 
to his own country poor in purse, but rich in knowledge. All his works ai'e lost ; 
but, according to the testimony of others, he was a great philosopher. He taught 
that the light of the Milky Way is caused by a countless number of stars crowded 
together, and that other planets would be discovered to belong to our system, 

Eudoxus, who lived b. c. 350, was distinguished for his knowledge of Astro- 
nomy, which it is said he acquired in Egypt, where he spent thirteen years in 
study under the priests. He was the author of some scientific works, which have 
been partially preserved in the writings of others of his countrymen. He built aa 
observatory at Cnidus, which Strabo speaks of as partly in ruins in his time. 

Pytheas, who lived about 380 b. c, was a celebrated astronomer of INIarseilles, 
or, as it was then termed, Massilia. He travelled as far north as the northern 
part of Norway, or perhaps Iceland, and discovered the great diminution of the 



326 bouvier's familiar astronomy. 

lengths of the nights at the summer solstice. His accounts were at first deemed 
fabulous, but modern astronomers consider him as the first person who taught us 
to distinguish climates by the difference in the length of the days and nights. 

In about a century after Meton discovered the cycle of 19 years, Calippus found 
that in order to make allowance for the hours by "which 6940 days are greater 
than 19 years, a day should be left out at the end of 76 years, or four Metonic 
cycles. This is called the Calippic period, and is an improvement on the cycle 
of Meton. 

Timocharus and Aristillus, who flourished about 300 b. c, made many import- 
ant astronomical observations. They observed and recorded the relative positions 
of the fixed stars, the places of the planets, and the times of the solstices, during 
a period of at least 25 years. 

The first person who attempted to measure the relative distances of the Sun and 
Moon was Aristarchus, of Samos. He also taught the stability of the Sun and 
the revolution of the Earth, for which he was accused of impiety. 

Appolonius, of Perga, in order to account for the motions of the heavenly 
bodies, is said to have introduced the complicated system of epicycles into the Gre- 
cian astronomy. These epicycles were imaginary revolving circles, having their 
centre within a larger circle, which in their rotations were supposed to carry the 
planets with them. Although this theory of Appolonius is now exploded, yet it 
could not have been formed without considerable knowledge of the movements 
of the planetary bodies. 

Posidonius estimated the distances of the Sun and Moon from the Earth : that 
of the Sun to be five hundred million stadia, and that of the Moon two million 
stadia. The atmosphere he computed to be four hundred stadia in height, which 
is not very far from the present estimate. 

Eratosthenes, a native of Cyrene, and second librarian of the Alexandrian Li- 
brary, lived about 200 years before the Christian era. He observed the obliquity 
of the ecliptic by means of the shadow of a style at Alexandria. He made use of 
two armils — instruments formed of brass circles — which were placed in the portico 
of the Square Porch at Alexandria, and were long used for the purpose of obser- 
vations of the Sun, &c. 

At the city of Syene, situated in Ethiopia, under the Tropic of Cancer, was a 
well, the bottom of which was said to have been illuminated on the day of the 
summer solstice. Eratosthenes supposed the cities of Alexandria and Syene to be 
on the same meridian, and by certain measurements by means of a style or 
gnomon, he computed the distance between the two cities to be one-fiftieth part 
of the circumference of the whole Earth. According to Noriet, a French astro- 
nomer under Napoleon, the measurement of Eratosthenes is too small by twelve 
minutes. 

But the most indefatigable of the ancient astronomers was Hipparchus, who 
lived 150 years before the birth of Christ. He discovered the precession of the 
equinoxes, or very slow apparent motion of the fixed stars from east to west. He 
made this discovery by comparing the observations of Timocharus and Aristillus, 
made about 150 years before, with his own. He was the inventor of many astro- 
nomical instruments, by means of which he noted the apparent magnitudes and 



I 



HISTORY OF ASTRONOMY. 6'J.l 

places of the heavenly bodies. He also rectified the length of the tropical year 
of ^65 days 6 hours, to within less than 5 minutes of the truth, as found by mo- 
dern observers. Having observed a star which he thought was a stranger to him, 
he resolved to make a catalogue of all which were visible, so that if any changes 
should take place in the configurations of the heavens, future generations might 
be able to detect them. His catalogue consisted of more than 1000 stars, which 
he traced on an artificial globe, and which was afterwards placed in the Alexan- 
drian Library. Delambre says Hipparchus appears to be the author of every 
great step in ancient astronomy. 

From the time of Hipparchus until the Christian era, the science of Astronomy 
was but little cultivated. 

Julius Caesar, who was born 100 b. c, was a benefactor to the science by re- 
modelling the calendar. He was well acquainted with Astronomy, and invited 
Sosigenes, an Egyptian mathematician and astronomer, to assist him in his 
labors. They fixed the length of the year to 365 days 6 hours. This is called 
the Julian Year. 

SECTION 11. 

^j^t gtstronomkal Instruments of tlje gincieitts, from l^e Earliest €mzB to 
l^e Christian ®ra. 

That the ancients made some tolerably accurate observations on the situations, 
magnitudes, and movements of the heavenly bodies, appears certain from the 
astronomical tables and other works which have come down to us. 

Perhaps the altitude of the Sun was the earliest measurement of angular dis- 
tance. This was determined by means of a gnomon, the invention of which is 
attributed to the Babylonians by Herodotus ; but it was also used by the Indians, 
Chinese, and Egyptians. 

This instrument consisted of a perpendicular staff or pillar, situated in a place 
exposed to the Sun, so that the shadow of the staff might be measured at various 
times during the year ; the difference of the length of the shadow served to deter- 
mine the altitude of the Sun. 

The gnomon was used by the Greeks at a very early period. It was sometimes 
made with a small hole in the top, to permit the rays of the Sun to pass through, 
and by that means to determine more accurately the length of the shadow. It is 
also supposed that the ancient Egyptians, and other Eastern nations, used this 
instrument to find the cardinal points. It is inferred that the pyramids and 
pagodas were placed due east and west by means of the gnomon. 

Thales erected a gnomon at Sparta, and set up the first sun-dial at Lacedaemon, 
by means of which he discovered the times of the equinoxes and solstices. 

In 1467, a gnomon was erected at Florence, by Toscanelli, which was 277 feet 
high. Another, 80 feet in height, was formerly erected in Paris. 

A very ancient instrument for measuring angles was the hemisphere of Borosus. 
It consisted of a hollow hemisphere with a horizontal rim, with a style placed in 
such a manner that the extremity of the style was in the centre of the sphere. 
The shadow of the style indicated the altitude of the Sun. This instrument served 



328 bouvier's familiar astronomy. 

rather for dividing the day into equal portions of time, than for determining 
positions. 

Instruments composed of circles were used for measuring angles. They had a 
border or limb, which was divided into equal parts, and these again subdivided. 

About the year 200 b. c, Ptolemy Euergetes caused two armils [armillce) to be 
erected in the portico at Alexandria. These instruments consisted of a circular thin 
plate of metal, so placed that the edge should coincide with the direction of the 
equator. Therefore, the Sun would shine under the plate when he was south of the 
equator, and over the plate when he was north of it. The moment when his raya 
would fall directly on the thin edge, and neither above nor below it, would desig- 
nate the time of the equinox. This instrument was called an equinoctial armil. 
Observations taken in this manner must necessarily be very inaccurate, owing to 
the refraction. 

The armil was graduated into 360°, and each degree into six parts of 10'' each. 

Ptolemy Claudius describes the solstitial armil as consisting of two concentric 
rings, one sliding within the other. The inner one was furnished with two pegs, 
exactly opposite to each other. These circles were fixed in the plane of the meri- 
dian, and the inner one turned till the shadow of one peg would fall exactly upon 
the other. The position of the Sun at noon would be determined by the degrees on 
the outer circle. The angles measured by these instruments would be expressed 
by parts of the whole circumference of a circle. 

It was soon found to be unnecessary to use the entire circumference of the circle 
for the purpose of making angular observations. Ptolemy employed an instru- 
ment somewhat resembling the mural quadrant of modern times. A quarter of a 
circle was described on a piece of wood or stone, which was divided into degrees 
and parts of a degree. This instrument was placed in the direction of the meri- 
dian, and in the centre of the arch a cylinder of wood was fixed perpendicular 
to the plane of the instrument, so as to project its shadow upon the limb of the 
quadrant, thus designating the altitude of the Sun or Moon. At the extremity 
of the radius, below the cylinder, was another cylinder fixed in a vertical position, 
which is supposed to have been used with the plumb-line to adjust the instrument 
to the horizon. Theon, a celebrated mathematician, states that a horizontal posi- 
tion was given to the quadrant by means of a water-level. 

The dioptra was an instrument invented by Archimedes, and improved by Hip- 
parchus. It consisted of two rulers, each seven feet long, and movable round a 
common centre. A sight was fixed at the centre of motion, and one at the extre- 
mity of each rule. The diameter of the Sun or Moon was then comprehended be- 
tween the two sights, and the opening of the rulers, or the angles which they 
formed, was the measure of the diameter of the luminary. 

The Egyptians measured the angular diameter of the Sun or Moon in the follow- 
ing manner : — They observed the direction of the shadow of the gnomon of an equi- 
noctial dial on the day of the equinox, at the moment the upper edge of the Sun ap- 
peared above the horizon; and again, its direction when the lower edge became 
visible. The angle between these lines is that subtended by the diameter of the 
Sun. This method is liable to great error, for the Egyptians estimated the true 
diameter of the Sun three times greater than the truth. 



HISTORY OF ASTRONOMY. 329 

They measured the angular diameter of the Sun much more accurately by 
means of the clepsydra. It consisted of a vessel with several small openings at the 
bottom, through which the water contained in it was permitted to escape. The 
clepsydra was probably invented in Egypt under the Ptolemys, though some 
authors ascribe the invention to the Greeks, and others to the Romans. 

These instruments were employed in the following manner: — They measured 
the quantity of water which flowed during the rising of the Sun's disc on the day 
of the equinox, and compared it with the quantity which flowed during the 
whole day ; the proportion of the latter to the former was considered the same as 
that of the circumference of a circle to the arc which subtends the angular diame- 
ter of the Sun. By this method they obtained the angular measurement with 
tolerable exactness, considering the means employed to arrive at it. 

A most singular method for measuring the angular diameter of the Sun was 
proposed by Cleomedes. The moment the upper limb of the Sun was seen to ap- 
pear above the horizon, a horse was set off in a gallop on a plain ; and the moment 
the whole disc became visible, he was to be stopped. The distance run by the 
horse was found to be ten stadia ; and if the motion of the Earth were equal to 
that of the horse, he concluded the diameter of the Sun would be equal to ten 
stadia. This method is singularly absurd. 



CHAPTER II. 

SECTION I. 

J^ront i\z Cljristian ®ra to i\}t gear ^btecn P«ubrcb. 

Ptolemy, who flourished during the first half of the second century of our era, 
is noted for his Megale Syntaxis, or, as it was afterwards called, the Ahnagest. 
This work contains the principal discoveries of Hipparchus, and his catalogue of 
stars, which has been already noticed. This catalogue Ptolemy reduced to his 
own time, either by observation or by corrections for the precession of the equi- 
noxes, a discovery of Hipparchus which he confirmed. 

The theory of the system as taught by Ptolemy was, that the Earth is placed in 
the centre of the universe, and that the Sun, Moon, and planets move uniformly 
in circles, the centres of which circles move in regular circles round the Earth. 
The origin of this theory, -says Grant, belongs to a "much higher antiquity." 

The order of the planets, according to the Ptolemaic theory, is, the Earth in the 
centre, next the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn. 

The discovery of refraction is said to be due to Ptolemy, who made some rude 
experiments to explain its law, which were partially successful. He also ex- 
plained the diff'erence in the apparent magnitudes of the Sun and Moon when 
near the horizon. 

Another great discovery was made by Ptolemy, namely : the evection or second 
inequality in the Moon's motion. He also applied himself to the study of music, 
geography, and chronology ; and was, according to some, a believer in the science 
of astrology, which was studied by most of the ancients. Other authors deny 



830 bouvier's familiar astronomy. 

that Ptolemy lent his sanction to astrology, it never having gained admittance 
into his penetrating mind. The former opinion, however, is generally entertained. 

As Ptolemy was desirous of transmitting his herculean labors to posterity, he 
caused his systems of the universe to be engraven on stone and erected in the 
Temple of Serapis, at Canopus, according to the authority of Theodorus. 

Hypatia, the daughter of Theon II. of Alexandria, wrote one book of a com- 
mentary on the Almagest, the remaining part of which was written by her father. 
She was among the first female scientific writers. Among her works are some 
mathematical tables of her own construction. Her reputation for learning, and 
her purity of life, created a jealousy in the minds of some ; and in the year 415, 
she was assassinated by a mob in the streets of Alexandria. 

ASTRONOMY OF THE ARABIANS. 

From the time of Ptolemy, a period of 600 years elapsed without any advances 
having been made in this science. 

During an interval of about 1300 years, astronomical science was entirely in 
the possession of the Arabians, who received it from the Greeks. The Arabian 
astronomers made inconsiderable additions to those thus obtained ; some of them, 
however, have been handed down, and are in use at the present day. Instead of 
the sexagesimal arithmetic of the Greeks, the Arabs introduced the notation by 
means of digits, 1, 2, 3, &c., which in fact are of Indian origin. They also intro- 
duced some important mathematical inventions. 

The first Arabian astronomer of note was the Caliph Almansor, who reigned 
A. D. 754. He not only encouraged, but cultivated the sciences. His grandson, 
the great Harun-al-Raschid, began to reign in the year 786. Like his grand- 
father, he cultivated the sciences, especially Astronomy. In 799 he sent to 
Charlemagne a clepsydra, or water-clock, of very ingenious construction. "In 
the dial were twelve small doors, forming the divisions of the hours ; one of these 
doors opened at each successive hour and let out little balls, which, falling on a 
brazen bell, struck the hour. The doors continued open till twelve o'clock, when 
twelve little knights, mounted on horseback, came out together, paraded round 
the dial, and shut all the doors." 

Almamon, the son of Harun-al-Raschid, reigned at Babylon in 814. He studied 
the sciences from a Christian physician, named Musva, and used every means to 
inspire a love of learning in his subjects. He caused all the best writings of the 
Greeks, and in particular Ptolemy's Almagest, to be translated into Arabic. 

Almamon determined the obliquity of the ecliptic to be 23° 35'', or, according 
to Vossius, 23° 34^. 

During the reign of the Caliph Almamon, lived Albategnius, surnamed the Ara- 
bian Ptolemy, who made many valuable observations in Mesopotamia about the 
year 880. He wrote a work entitled De Scientia Stellarum, which included all 
then known in the science of Astronomy. It was afterwards translated into 
Latin. 

Ebn-Junis, who lived about the year 1000, was astronomer of Akim, the caliph 
of Egypt. He made observations at Cairo, proving that the mean motion of the 
Moon is subject to a small acceleration, which, accumulating after a long lapse 



HISTORY OF ASTRONOxMY. 831 

of time, must be admitted into astronomical calculations. He was the author of a 
set of tables which were long in use in the East. 

When the Arabs were settled in Spain, they built observatories in several cities, 
and paid great attention to the cultivation of Astronomy. 

In 1020, Arzachel distinguished himself by his researches and observations. 
He made a set of astronomical tables, entitled Tabulae Toledanse. 

In 1100, Alhazen, another Arabian astronomer, settled in Spain. He wrote a 
work on Optics, in which he explains the true cause of refraction and twilight. 
He also invented two theorems in spherical trigonometry, which are of great use 
in Astronomy. 

ASTRONOMY OF THE PERSIANS. 

The Persians, from the fifth century before the Christian era, were zealous 
observers of the heavenly bodies, in which they were encouraged by the example 
of many of their emperors. 

They devoted themselves to determining the length of the year, which they 
fixed to 365 days and 6 hours. But they did not count the 6 hours in every year, 
but made it to consist just of 365 days. In order to account for the 6 hours, they 
intercalated a month of 30 days in every 120 years, which is the same as inter- 
calating one day in every 4 years, according to the Julian method. As this was 
found to exceed the period of the Earth's revolution by about eleven minutes, 
Omar Cheyham, a Persian astronomer, undertook to rectify this error. He pro- 
posed to add a day every fourth year for seven periods of four years each, and 
then a day to the fifth year for the eighth period, and so on. This system, which 
is very near the truth, was adopted instead of the intercalary month above men- 
tioned, and has been retained in Persia to the present time. 

ASTRONOMY OF THE TARTARS. 

Genghis Khan, the founder of the Mogul Empire, was very fond of all the 
sciences, especially Astronomy, to the professors of which he showed the highest 
respect. Houlagou Khan, one of his descendants, about the year 1264 built an 
observatory in the city of Maragha, and invited a great number of astronomers to 
dwell there. He appointed Nassir Eddin as president of their society, and en- 
couraged every advancement in the study of Astronomy. Nassir Eddin wrote 
several works, among which are a Treatise on the Astrolabe, the Motions of the 
Heavenly Bodies, and some astronomical tables. 

About the year 1450, lived TJlugh Begh, grandson of Tamerlane, and prince of 
the Tartars, who was one of the most learned men of his age. He established an 
astronomical academy in Samarcand, the capital of his dominions, and furnished 
it with the best instruments which could be procured. 

Some of the works of Ulugh Begh are now in print, and some are in manuscript, 
in a few libraries. His best work is his Catalogue of the Stars and Astronomical 
Tables, which were the most perfect then known in the East. 

After the death of this prince. Astronomy was not studied to any extent in the 
East, and finally became so interwoven with Astrology as to be no longer identi- 
fied. The Persians of the present day have but a meagre knowledge of the science. 



332 bouvier's familiar astronomy. 



ASTRONOMY OF MODERN EUROPE. 

The Arabians preserved the knowledge of Astronomy, which they had derived 
chiefly from the Greeks, and in the practical department of the science displayed 
a striking superiority to their masters. 

In the ninth and tenth centuries, men of learning travelled into Spain for the 
purpose of studying Astronomy and Mathematics at the Moorish universities. 
About the year 1250, Alphonso X., King of Castile and Leon, founded at his capi- 
tal a college for the advancement of Astronomy. He caused a new set of astro- 
nomical tables to be published, which were known as the Alphonsine Tables. They 
were executed at an immense expense, under the superintendence of some of the 
most learned astronomers of that day, both Arab and Jewish. About this time 
Frederic II., Emperor of Germany, caused the works of Aristotle and Ptolemy to 
be translated into Latin. 

Ptoger Bacon, one of the most extraordinary minds of that or any age, made 
some valuable suggestions in the construction and use of astronomical instruments. 
He also proposed a reformation in the calendar three hundred years before any 
corrections were made in it. 

Nicholas Copernicus, who was born at Thorn, in Prussia, in the year 1473, is 
known as the restorer of the true system of Astronomy. Being gifted with rare 
sagacity, he soon learned' to distinguish the simple operations of nature from the 
intricate theories invented by man. 

About the year 1507, Copernicus became a convert to the system of Astronomy 
embraced by Pythagoras, but which had its origin at a period prior to the time of 
the latter philosopher. According to the theory of Copernicus, the planets move 
in circular orbits from west to east, in the following order — viz. Mercury, Yenus, 
Earth, Mars, Jupiter, and Saturn ; the Moon revolves round the Earth, and also 
accompanies the latter body round the Sun. 

Although he believed the orbits of the planets to be circular, he did not con- 
ceive them to be concentric. The Sun's place, according to his theory, was not 
in the common centre, but a position so placed with regard to them all as with 
the addition of some epicycles might account for all the observed phenomena. 
He was led to renounce the theory of the Earth in the centre of the system, be- 
cause all the exterior planets presented such variable appearances in the different 
parts of their orbits. He especially observed the variable brilliancy of Mars and 
Jupiter ; for when the former is in opposition to the Sun, he almost rivals Jupiter 
in brilliancy, while near conjunction he was no bi'ighter than a star of the second 
magnitude. This fact led him to conceive the idea that the superior planets re- 
volve around the Sun instead of around the Earth. 

The phenomena of the interior planets he found clearly explained in the Egyp- 
tian Astronomy, which ascribes to the planets Mercury and Venus orbits having 
the Sun for a centre. This theory, Copernicus found, would account for the appa- 
rent oscillation of those planets on each side of the Sun. 

Some of the ancient philosophers had supposed the fixed stars to be at rest, and 
explained their apparent diurnal motion from east to west by ascribing to the 
Earth a revolution on its axis in the contrary direction ; that is, from west to 



HISTORY OF ASTRONOMY. 333 

east. Copernicus embraced this doctrine of the Earth's rotation, and thus com- 
bined all these fragments of truth into a system which accounted more clearly 
than any of its predecessors for the phenomena observed in the motions of the 
heavenly bodies. 

The doctrine that the Earth is the centre of the system, so strenuously main- 
tained during the 1400 years which elapsed from the time of Ptolemy, was com- 
pletely refuted by Copernicus, who revived the true theory of planetary motions ; 
yet it must be understood that the system he promulgated, and which is distin- 
guished by his name, is not strictly that which is known as the true planetary 
system at the present day. In 1543, Copernicus published his work, "Z>e Revo- 
lutionibus Orbium Coe.lestium,^' explaining the solar system, with the Sun in the 
centre. He lived only a few hours after he received the first copy. 

After the death of Copernicus, the science of Astronomy received a fresh im- 
pulse from the researches of Nonius, Appian, the Landgrave of Hesse, and many 
others. Nonius was the inventor of some astronomical instruments, among which 
is an improvement on the divisions of the limb of the quadrant then in use. He 
also made many valuable discoveries in the sciences of mathematics and navi- 
gation. 

About 1560, William IV., Landgrave of Hesse-Cassel, built an observatory in 
his capital, and furnished it with the best instruments of that day. He made 
many observations on the fixed stars, a catalogue of which was formed by Snel- 
lius. His catalogue consisted of 400 stars. 

Contemporary with the Landgrave of Hesse, and in close intimacy with him, 
lived Tycho Brahe, one of the greatest observers that ever lived. He was born in 
Denmark, in the year 1546. He would not acknowledge the truth of the Coper- 
nican system, though it must have been evident to a mind like his, that it was 
more in accordance with the observations of the planetary motions than any other 
then known. Yet, although Tycho could not embrace the theory of Ptolemy, his 
superstitious feelings would not permit him to endorse the doctrines of Copernicus. 
He therefore adopted a system of his own, which was, that the Earth is immovably 
fixed in the centre of the system, about which first revolved the Moon, and then 
the Sun, which carried with it in its sphere of rotation the planets Mercury, Ve- 
nus, Mars, Jupiter, and Saturn. This is known as the Tychonic system. 

Tycho Brahe constructed some astronomical instruments on a much larger scale 
than were ever made before ; by means of which he applied himself diligently to 
celestial observations. He discovered the variation of the Moon, and determined 
the inclination of the lunar orbit with more precision than had ever been done 
before. 

Tycho denounced the then prevalent theory that comets wei'e only transient 
meteors, and demonstrated them to be solid bodies, revolving round the Sun at 
stated periods. 

In the year 1572, he observed a new star in the constellation Cassiopeia, which 
induced him to make a new catalogue of stars. He built an observatory on the 
island of Huen, under the patronage of Frederick 11. , King of Denmark, which he 
called Uranibourg, or the (7% of the Heavens. 

The next noted astronomer which appeared was Galileo, who was born at Pisa, 



334: bocvier's familiar astronomy. 

in 1564:. While yet a student at the university, he was one day in the cathedral 
at Pisa, and, perceiving a lamp which was suspended from the ceiling swinging to 
and fro, he was impressed with the idea that the lapse of time might be measured 
by a pendulum. 

Daring the latter end of this century, Galileo made many experiments on the 
velocity of falling bodies, which, being opposed to the pop\ilar theories as taught 
by the followers of Aristotle, required no ordinary degree of sagacity to over- 
throw ; for they were so thoroughly interwoven with many of the other doctrines 
of that great philosopher, as to render it at once difl&cult and unpopular to attempt 
to destroy that which had apparently stood the test of ages. The strongest pre- 
judices were roused against him, so that in 1592 he quitted Pisa, though not be- 
fore clearly proving the truth of his own theories by actual experiments made 
from the Leaning Tower of that city. 

About the year 1600, he succeeded in constructing a telescope, with which he 
discovered the satellites of Jupiter, inequalities in the surface of the Moon, innu- 
merable fixed stars, and a ring, or as he then described it, handles, (ansae,) to the 
planet Saturn. These discoveries he announced in the Sidereal Messenger, which 
was published in Venice in 1610. 

While Galileo was pursuing his discoveries in Italy, a star arose in Germany 
whose light was destined to shine throughout all future ages. This was John 
Kepler, who was born at Wittemburg, in Germany, in 1571. He adopted the 
theory of Copernicus, and discovered the cause of the tides in the ocean. His 
great work is De Motibus Stellce JIartis, in which he proves that Mars moves in an 
elliptical orbit, in one of the foci of which the Sun is placed. Upon this discovery 
he established his first and second laws of planetary motion — viz. 1. All the 
planets move in ellipses, having the Sun in one of the foci ; 2. A line drawn from 
a planet to the Sun sweeps over equal areas in equal times. He discovered the 
fourth inequality, or annual equation of the Moon ; he also supposed the existence 
of a planet between the orbits of Mars and Jupiter, too small to be visible to the 
naked eye, and that the Sun has a revolution on its axis ; which facts have been 
since proved. 

Descartes, born in 1596, at Touraine, was one of the greatest geniuses of the 
seventeenth century, although, perhaps, his labors did not promote the progress 
of Astronomy to as great a degree as some of his contemporaries whose minds 
were less liberally endowed. His hypothesis of ethereal vortices is considered by 
some to have paved the way for the mechanical theory of planetary motions ; but 
Delambre has justly remarked that, by misleading men's minds from nature, this 
fiction of the imagination retarded, rather than promoted, the progress of true 
science. Descartes made important discoveries in mathematical science, and did 
much to overthrow the Aristotelian philosophy. 

About the same period, and contemporary with Descartes, flourished Huyghens, 
a philosopher gifted with equal genius, but possessing greater discrimination and 
caution. He is distinguished for his telescopic observations, among which are his 
discovery that the appendage with which Saturn is furnished is a luminous ring, 
and also detected the most conspicuous satellites of that planet. 



HISTORY OF ASTRONOMY. 335 



SECTION II. 

gistronomkal ^mtxnmmtB |nhnttJ^ from t^t €^rbtia« €ra to il^t gjar 

We have but little handed down to us with regard to the means used by the 
ancients to determine the various phenomena, some of which are so accurately 
recorded. All we know of their astronomical instruments is, that they were much 
less accurate than those of our day. 

About the year 1830, when Sir John Malcolm, formerly governor of Bombay, 
visited Maragha, in Persia, he traced distinctly the foundations of the observatory 
constructed in the 13th century for Naser-ood-Deen, the favorite philosopher of 
the Tartar prince Hoolakoo, or Ilulagou, the grandson of Genghis Khan, who, in 
this locality, relaxed from his warlike toils, and assembled around him the most 
learned men of the age ; who, having commemorated his love of science, have 
given him more fame as its munificent patron, than he acquired by all his con- 
quests. 

In this observatory, according to one of the best Mohammedan writers, there 
was a species of apparatus to represent the celestial sphere, the signs of the zodiac, 
and the conjunctions, transits, and revolutions of the heavenly bodies. Through 
a perforation in the dome, the rays of the Sun were admitted so as to strike upon 
certain lines on the pavement, indicating in degrees and minutes the altitude 
and declination of that luminary during every season, and to mark the hour of the 
day throughout the year. The observatory was further supplied with a map of 
all the knoAvn parts of the terrestrial globe, as well as a general outline of the 
ocean, with all the islands then discovered. According to a Mohammedan author, 
all these were so arranged and delineated as "to remove, by the clearest demon- 
stration, every doubt in the mind of the student." 

There is a story extant, though of doubtful authenticity, that Ulugh Begh 
caused an enormous quadrant, of 180 feet radius, to be constructed, with which he 
made observations. The management of such an unwieldy instrument seems 
physically impossible. Certain it is, however, that those astronomers found the 
latitude of Samarcand, and fixed the obliquity of the ecliptic to but a fraction more 
than two minutes from that found by modern observers. 

Towards the end of the thirteenth century, Wallingford, Abbot of St. Albans, 
made a clock for his convent, which indicated the hours, the courses of the Sun 
and Moon, the time of high water, &c., the manuscript account and description of 
which are preserved in the Bodleian Library. 

In the fifteenth century, Walther, of Nuremburg, was the first who made use of 
clocks in his astronomical observations And to Huyghens are we indebted for the 
application of pendulums to clocks, although mechanical constructions with weights 
had been used to measure time as early as the thirteenth century. Galileo also 
conceived the idea of using the pendulum, but could not accomplish his scheme, 
because he endeavored to make the pendulum the prime mover. 

In the sixteenth century Nonius invented some useful instruments, and greatly 
improved others, among which are the astronomical quadrant. He divided this 



336 bouvier's familiar astronomy. 

instrument into degrees and parts more accurately than had before been conceived 
possible. 

The instruments used by Tycho Brah^, in his observatory at Huen, are described 
as very complete. The quadrants were so accurately divided that the arc might 
be read off true to one-sixth of a minute. One of his quadrants was divided ac- 
cording to the method of Nonius. 



CHAPTER III. 

SECTION I. 

Jrom i^t ^egmnhtg of lljc ^thntt^nl^ to t^e €nb of tlje ^igl^tcfntlj C^ntnrg. 

On the death of Tycho Brah^, in 1601, Kepler succeeded him as principal 
mathematician to the emperor. In 1606, he published a work on the optical part 
of Astronomy, entitled a "Supplement to Vitellius;" in 1609 appeared his New 
Astronomy, or De Motibus Stellce 3Iartis ; and in 1611 he published his Dioptrics, 
in which he gives the theory of the telescope. 

In 1618 he published his work entitled Harmonices Mundi, dedicated to James I. 
of England. This work contains his third law — " The squares of the periodic times 
of the planets are in proportion to the cubes of their mean distances from the 
Sun." Kepler had been seventeen years in search of this law, and when lie dis- 
covered it, he was almost frantic with joy. " The die is cast," he exclaimed, "the 
book is written to be read either now or by posterity, I care not which. It may 
well wait a century for a reader, as God has waited 6000 years for an observer." 
In 1628 Kepler finished the Eudolphine Tables, founded on the laws which he dis- 
covered, and the observations of Tycho Brahe. 

While Kepler was making his discoveries in the north, Galileo was no less occu- 
pied in the south of Europe, in his philosophical experiments. In the year 1609, 
while on a visit to a friend in Venice, he learned that a Dutchman of the name of 
Jansen had made an instrument which enabled persons to see at a greater distance 
than by the naked eye. By means of a telescope, which magnified thirty times, 
lie discovered that the Moon is diversified with mountains and caverns ; that the 
Sun, instead of being a ball of fire, is covered with irregularly-shaped black spots, 
and that it revolves on its axis ; that the planet Venus exhibits phases resembling 
the Moon ; that Jupiter is accompanied by four small stars or moons ; and that 
Saturn, unlike all the others, is not round, but has two handles, as it were, at its 
sides, and which he afterwards thought were two small planets, one on each side 
of the large one. These handles, or ansae, as they were called, were found to be 
the extremities of the ring, which Galileo's telescope was not sufficiently powerful 
to reveal. In the year 1617 he was charged with teaching the doctrine of the 
stability of the Sun and the annual motion of the Earth, which, being considered 
contrary to the Scriptures, caused the persecution and imprisonment of its author. 

The telescopic discoveries of Galileo exercised such a withering influence upon 
the ancient philosophy, that many of its adherents refused to acknowledge the 
truths thus revealed to them. " Oh ! my dear Kepler," says Galileo in one of his 



HISTORY OF ASTRONOMY. 337 

letters, "how I wisli we could have one hearty laugh together! Here, at Padua, 
is the principal professor of philosophy, whom I have repeatedly and earnestly 
requested to look at the Moon and planets through my glass, refuses to do so. 
What shouts of laughter we should have at this folly, and to hear the professor of 
philosophy in Pisa laboring before the grand duke with logical arguments, as with 
magical incantations, to charm the new planets out of the sky !" 

The last telescopic observations of Galileo resulted in the discovery of the diur- 
nal libration of the Moon. He was attacked soon after with a disease of the eyes, 
which in a few months rendered him totally blind. This sudden and severe 
calamity almost overpowered him, as he was thereby incapacitated for any future 
observations of the heavens. In writing to a friend, he says — "Alas! your dear 
friend and servant has become totally and irreparably blind. These heavens, this 
Earth, this universe, which, by powerful observation I had enlarged a thousand 
times beyond the belief of past ages, are henceforth shrunk into the narrow 
space which I occupy myself. So it pleases God ; it shall, therefore, please 
me also." 

In the early part of this century. Lord Napier invented logarithms, " which," 
says La Place, "by reducing the labors of months to a few days, doubles the life 
of the astronomer." 

About the same time, Scheiner, a German astronomer, made numerous observa- 
tions on the solar spots ; and Bayer, of Augsburg, published a map and descrip- 
tion of the constellations. 

The elder Cassiai was the contemporary of Huyghens, and a noted astronomer. 
He was the first professor in the Royal Observatory of Paris, and belonged to a 
family for more than 250 years illustrious in the scientific annals of France. Cas- 
sini constructed the first tables of Jupiter's satellites which were deserving of 
confidence. He discovered four of Saturn's satellites, the belts of Jupiter, and 
his rotation on his axis, and made many other important additions to science. 
Harriot also flourished about this time, to whom we owe many contributions to 
astronomical and mathematical science. 

Horrox, a young English astronomer, in the year 1639 witnessed the first transit 
of Venus ever seen by man. Gassendi observed a transit of Mercury eight years 
previous. 

One of the most indefatigable observers of those times was Hevelius, a rich citi- 
zen of Dantzic, who devoted his life and fortune to the service of his favorite 
science. He made drawings of the different phases of the Moon, which cost him 
years of labor. He also made numerous observations on comets, and concluded 
that as their orbits could not be circular, they must move in parabolas. 

Huyghens added his share to the large stock of astronomical knowledge which 
was now accumulating through the exertions of some of the brightest intellects 
which the world has ever produced. Besides inventing some instruments, he dis- 
covered the ring of Saturn, and one of the satellites. As the number of satellites 
just eqtialled the number of planets then known, Huyghens considered it unne- 
cessary to seek for any more, as the equality was necessary for the harmony of 
nature. This superstitious notion was common in those times, as may be seen by 
the writings of Kepler and others. 



338 bouvier's familiar astronomy. 

About this time Roemer discovered the progressive motion of light, and mea 
sured its velocity by means of the eclipses of Jupiter's satellites. 

As astronomical knowledge had recently received such valuable accessions in 
the labors and inventions of those men of genius who flourished during the pre- 
ceding half century, it was deemed necessary to erect observatories, and furnish 
them with all the best apparatus, in order not only to encourage the increasing 
taste for this branch of science, but also to facilitate celestial observations. In 
the year 1670, the Royal Observatory of Paris was completed, and Dominic Cassini 
was invited by Louis XIV. to take charge of it. He enriched science by a number 
of valuable observations and discoveries, among which were tables of the motions 
and eclipses of Jupiter's satellites, the double ring of Saturn, and four more satel- 
lites belonging to that planet. He determined the rotation of Jupiter and Mars, 
besides many other discoveries : he made the first table of refractions. Cassini 
devoted his life chiefly in observing the appearances and motions of the heavenly 
bodies, which finally so affected his sight as to render him blind. The charge 
of the Paris observatory remained in the Cassini family during the period of 
120 years. 

The very year in which Galileo died, the great Newton was born. He dis- 
covered the cause of the precession of the equinoxes, and determined some of the 
principal lunar inequalities and planetary perturbations. In the year 1666, it is 
said, Newton first conceived the idea of gravitation from seeing an apple fall from 
a tree. This led him to attribute the retention of the Moon in her orbit to the 
same cause ; a fact which his future calculations tended to confirm. He observed 
that the force of gravitation on the summits of the highest mountains is nearly the 
same as at the level of the sea ; from which he inferred that the attractive influ- 
ence of the Earth extends to the Moon, and, combining with her projectile force, 
retains her in her orbit round the Sun. Hence followed the inference that if ter- 
restrial gravitation retains the Moon in her orbit, solar gravitation must be the 
principle which influences the planets, when combined with their projectile forces, 
to perform their revolutions round the Sun. This theory he proved to be true by 
showing the areas are proportional to the times. 

When in his 30th year, Newton was proposed as a FeUow of the Royal Society. 
On being informed of the fact, he remarked to the secretary that he hoped he 
would be elected, and added that he *' would endeavor to testify his gratitude by 
communicating what his poor and solitary endeavors could effect towards pro- 
moting their philosophical designs." Soon after his election, he communicated to 
the society a paper on the composition and decomposition of light, and his theory 
of colors. About this time he resided next door to a widow lady, who was much 
puzzled by the peculiarities of the philosopher. On one occasion, a Fellow of the 
Royal Society happening to call upon her, she remarked that "in the adjoining 
house a poor crazy gentleman had come to reside, who diverts himself in the oddest 
ways imaginable. Every morning, when the Sun shines so brightly that we are 
obliged to close the window blinds, he takes his seat in front of a tub of soap-suds, 
and occupies himself for hours blowing soap-bubbles through a common pipe, and 
watching them intently till they bvirst. He is doubtless now at his favorite 
amusement," she added; "do come and look at him." The gentleman, looking 



HISTORY OF ASTRONOMY. 339 

into the adjoining yard, turned round and said — " My dear madam, the person 
whom you suppose to be a poor lunatic is no other than Newton, the great 
philosopher, studying the refraction of light upon thin plates, a phenomenon 
which is beautifully exhibited upon the surface of a common soap-bubble." This 
anecdote serves as an excellent moral, teaching us not to ridicule what we do not 
understand, but gently and industriously to gather wisdom from every circum- 
stance around us. 

Newton was the author of many works, the principal of which was his Frincipia, 
which is destined to stand as a monument of his genius. The following remarks 
by Mr. Whewell tend to enhance the admiration and wonder with which the im- 
mortal discoverer of universal gravitation will always be regarded : — " No one, for 
sixty years after the publication of the Frincipia, and with Newton's methods, — no 
one, up to the present day, has added any thing of any value to his deductions. 
We know that he calculated all the principal lunar inequalities ; in many of the 
cases he has given us his processes, in others only his results. But who has pre- 
sented in his beautiful geometry, or deduced from his simple principles, any of the 
inequalities which he left untouched ? The ponderous instrument of synthesis, so 
effective in his hand, has never since been grasped by one who could use it for 
such purposes ; and we gaze at it with admiring curiosity, as on some gigantic 
implement of war which stands idle among the memorials of ancient days, and 
makes us wonder what manner of man he was who could wield as a weapon what 
we can hardly lift as a burden." This immortal genius, who contributed proba- 
bly more than any other man to the extension of human knowledge, felt no differ- 
ence between his own mind and that of other philosophers. In a letter to a friend, 
he says — " If I have done the public any service, it is due to nothing but industry 
and patient thought." It is said of him, that while preparing his Frincipia, he 
lived only to calculate and think. 

The Greenwich Observatory was erected in 1675, and Flamsteed was appointed 
to the charge of it, which office he filled for upwards of 30 years. He published 
a catalogue of 2284 stars, made many observations on the fixed stars, planets, 
comets, and spots on the Sun, &c. His labors were chiefly confined to the prac- 
tical part of Astronomy. 

Edmund Halley, the son of a wealthy citizen of London, from his earliest years 
devoted himself to the study of Mathematics and Astronomy. While only in his 
19th year, he made some discoveries with regard to the orbits of the planets, and 
soon after undertook a voyage to the island of St. Helena, in order to make a cata- 
logue of stars in the southern hemisphere. He revived the old theory that comets 
belong to the solar system, and ventured to predict that the comet of 1681 would 
return in the year 1759, which was verified; the return having occurred 17 years 
after his death. In advising astronomers to watch for its reappearance, Halley 
expressed a hope that in the event of its return they would acknowledge its pe- 
riodicity had been discovered by an Englishman. 

On the death of Flamsteed, in 1719, Halley was appointed to fill his office in the 
Greenwich Observatory. It is to Halley that posterity is indebted for Newton's 
Frincipia; for had it not been for his earnest solicitations, Newton would not 
have given that great work to the world. 



340 bouvier's familiar astroxomy. 

Towards the close of the seventeenth century, Bradley, the successor of Halley, 
astonished the world by his sagacious observations. There had been an annual ap- 
parent motion observed in the polar and circumpolar stars by many astronomers, 
which had never been satisfactorily explained. Bradley, at last, after much 
deliberation, determined it to be owing to the motion of light combined with the 
annual motion of the Earth. This discovery, called the aberration of light, was 
made in the year 1728. He afterwards discovered nutation, which, with the 
former discovery of aberration, entitles him to a high place among the list of 
astronomers. 

While Bradley was distinguishing himself in England, Lacaille was laboring 
successfully in the same cause in France. In 1751 he made a voyage to the Cape 
of Good Hope for the purpose of determining the Sun's parallax, and to form a 
catalogue of the southern circumpolar stars. He observed more than ten thou- 
sand stars situated between the Tropic of Capricorn and the pole, of which he com- 
puted the places of 1942. This herculean task was performed in the incredibly 
short period of one year. He also made many other important astronomical ob- 
servations. 

Another genius of extraordinary powers now appeared on the stage. This was 
William — afterwards Sir William — Herschel. He rendered his name immortal by 
the discovery, in the year 1781, of another member, beyond the limits formerly as- 
signed to our system. This body was first named Georgium Sidus, in honor of his 
patron, but the name of Uranus is now preferred. By means of more pow- 
erful telescopes than any then known, he discovered the belts of Saturn, and two 
more satellites belonging to that planet. His observations on double stars and 
nebulae have opened a new field for the astronomer. He was assisted in his labors 
by his sister. Miss Caroline Herschel, whose name should be enrolled among the 
observers of that day. 

Miss Herschel was the constant companion of her brother, and the sole assist- 
ant of his astronomical labors, to the success of which, her indefatigable diligence 
and singular accuracy of calculation greatly contributed. In the intervals, she 
found time for observations and discoveries of her own, among which were no less 
than eight comets, besides many nebulae and clusters previously unobserved. As 
a reward for these labors. King George III. placed her in receipt of a salary quit© 
sufficient for her singularly moderate wants and retired habits. 

In speaking of Miss Caroline Herschel, Dr. Nichol says — "The astronomer. 
(Sir William Herschel,) during these engrossing nights, was constantly assisted 
in his labors by a devoted maiden sister, who braved with him the inclemency of 
the weather — who heroically shared his privations, that she might participate in 
his delights — whose pen, we are told, committed to paper his notes of observations 
as they issued from his lips ; she it was who, having passed the nights near the 
telescope, took the rough manuscripts to her cottage at the dawn of day, and pro- 
duced a fair copy of the night's work on the ensuing morning ; she it was who 
planned the labor of each succeeding night, who reduced every observation, made 
every calculation, and kept every thing in systematic order ; she it was — Miss 
Caroline Herschel — who helped our astronomer to gather an imperishable name. 
* * * Some years ago the gold medal of our astronomical society was trans- 



HISTORY OF ASTRONOMY. 341 

mitted to her at her native Hanover, whither she removed after Sir William's 
death ; and the same learned society has recently inscribed her name upon its 
roll. But she has been rewarded by yet more — ^by what she will value beyond all 
earthly pleasures ; she has lived to see her favorite nephew — him who grew up 
under her eye unto an astronomer — gather around him the highest hopes of 
scientific Europe, and prove himself fully equal to tread in the footsteps of his 
father." — Dr. NichoVs Architecture of the Heavens. 

Miss Herschel died at Hanover in the year 1848, in the 98th year of her age. 

Contemporary with Herschel, lived Piazzi, at Palermo, where he established an 
observatory, which was furnished with the best instruments. He was an accurate 
observer, and made for himself a great name. 



SECTION II. 

gistrottomkal |nstrumculs irt itse from i\z fegmnmg d i\z %thzxdzzxd\ to i\z 
€nb of l^e @x0^tant^ Centurg. 

The epoch under consideration is a period noted for the improvements and in- 
ventions of astronomical instruments. 

The collection of instruments at Uranibourg, belonging to Tycho Brahe, was 
the finest which had ever been made. His mural quadrant, of five cubits radius, 
was noted for its minute graduation. 

Cassini erected a gnomon in the church of St. Petronius, at Bologna, which was 
83 French feet in height. 

The first telescope made by Galileo, in the early part of the seventeenth century, 
was formed by the combination of a plano-convex and plano-concave lens fitted in 
a leaden tube. This instrument magnified only about three times. He made 
another, which magnified about eight times, which giving him encouragement for 
further effort, he succeeded in making one with a power of more than thirty. 
With this telescope he made those observations mentioned in the first part of this 
chapter. 

Huyghens directed his attention most particularly to optics. He made a tele- 
scope of ten feet, by means of which he discovered one of the satellites of Saturn. 
He applied the pendulum to clocks, which may be considered as a valuable gift 
conferred on the science of Astronomy. He was the first to determine what 
should be the length of a pendulum whose vibrations should each be performed in 
a second of time. 

Gascoigne originated some improvements in practical Astronomy. He was the 
first person who introduced the use of telescopic sights, and was the inventor of 
the micrometer. But to Huyghens, Malvasia, and Azout are we indebted for the 
application of the micrometer to the telescope. 

Picard introduced the application of telescopes and micrometers to graduated 
instruments. In 1669, he constructed a quadrant, to which he applied an astro- 
nomical telescope, which he made use of to observe the stars on the meridian in 
the daytime. This telescope had cross-wives in its focus. But Huyghens had 
invented the plate micrometer nearly twenty years before. 



342 bouvier's familiar astronomy. 

These great improvements in instruments gave an impulse to astronomical dis- 
covery which had not been known before, and which threatened to overthrow old, 
preconceived notions. Hevelius refused to make use of them, because they would 
make all the old observations of little value, and render useless the results of a 
long life of arduous labor. 

Picard was the first person who determined the right ascensions of the stars by 
observing their meridian transits. In these observations he always made use of 
the pendulum. 

In the year 1644, Roemer, a Danish astronomer, introduced the method of ob- 
serving the right ascension of celestial bodies by means of a transit instrument ; 
that is, a telescope attached to a horizontal axis perpendicular to its length, and 
movable only in the plane of the meridian. 

Graham executed a large mural arc for Halley, at Greenwich ; and also a sector 
for Bradley, with which he detected aberration. 

Molyneux and Bradley commenced a series of observations in 1725, with a 
zenith sector whose radius was 24 feet. 

Astronomy received a strong impulse during this period by the erection of public 
observatories — the first of which is the Boyal Observatory of Copenhagen, in 1656; 
second, the Royal Observatory of Paris, in 1667; after which is the Royal Ob- 
servatory at Greenwich, in 1675. To James Gregory we owe the invention of that 
form of the reflecting telescope which bears his name. In 1786, Louis XYI. 
directed !M. de Beauchamp to superintend the erection of an observatory at Bag- 
dad, which bore the inscription — " Built to restore the Chaldean and Arabian ob- 
servations." 

About the year 1767, Bird, an English artist, divided several quadrants for 
public observations, which were so accurately done that the English government 
purchased from him the secret. 

Ramsden, another artist, was also very successful. He made a quadrant, which 
was sent to Padua, the greatest error of which never exceeded two seconds. In 
1788, Ramsden made a mural circle for M. Piazzi, at Palermo, which was five feet 
in diameter ; and one of eleven feet in diameter for the Dublin Observatory. 

Huyghens, in his first attempt at constructing telescopes, made one 22 feet long. 
Bat this was eclipsed by those of Campani, who, by order of Louis XIY., made 
them of 86, 100, and 136 feet in length. After this, Huyghens constructed one 
of no less than 210 feet in length. Azout and Hartsoecker are said to have made 
an object-glass of 600 feet focus, which certainly must have been unmanageable. 
Some of the object-glasses of Huyghens and Cassini were placed upon a pole, and 
the observer seated himself at the focus with an eye-glass, there being no tube to 
the telescope. 

The principal improvement in the telescope was the construction of lenses 
formed of glass possessing difi'erent degrees of refractive power, in which the 
chromatic aberration was almost entirely removed, and the spherical aberration 
materially diminished. This invention, called the achromatic telescope, is due to 
Mr. DoUond. Dollond and his son succeeded in constructing telescopes 3 feet 
long, with a triple object-glass, which produced an effect as great as those of 45 
feet on the old principle. 



HISTORY OF ASTRONOMY. 343 



CHAPTER IV. 

Pislorg of gtstronomg from ll^e g^ginnmg to t^c PibMe of t^t ^imkmi^ 

Centtirg. 

The first day of the nineteenth century was ushered in by the discovery of a 
new planet between the orbits of Mars and Jupiter. This body was discovered by 
Piazzi, at Palermo, on the 1st of January, 1801, and is named Ceres. The next 
year, 1802, witnessed the discovery of another small planet by Dr. Olbers, which 
he named Pallas. In 1804, Harding discovered a third in the same region, to 
which he gave the name of Juno ; and in 1807, Dr. Olbers discovered a fourth, 
which he called Vesta. These small bodies are known by the name of asteroids, 
only four of which were seen from 1807 until 1845, no other asteroids being then 
known to exist. By referring to the table of those bodies, and the dates of their 
discoveries, in Part II. Chapter V. Section II., a full description of them will 
be found. 

During the first half of this century, the attention of astronomers has been 
turned to comets ; many valuable observations have been made with regard to the 
orbits and movements of these bodies, a full account of which may be found m 
another part of this work. 

The discovery of a primary planet, yet more distant than Uranus, is another 
prominent feature in the discoveries of this century. 

M, Le Verrier, from theory alone, was enabled to fix on almost the precise spot 
where this body might be found, fully verifying the predictions so confidently as- 
serted, and triumphantly proving the certainty of mathematical calculations. 

The discoveries during this century are fully given under their proper heads in 
another part of this work, as well as the astronomers who have thus distinguished 
themselves. The observatories of Washington, Cambridge, and Cincinnati, as 
well as many in Europe, are doing honor to their respective countries by the 
means which they afford for the rapid advancement of the science of Astronomy. 

A description of the principal astronomical instruments now in use is given 
under Part IV. 

There is a galaxy of astronomers at the present day in Europe, as well as in 
our own country, to whom we are indebted for many valuable acquisitions to our 
fund of astronomical knowledge. The names of Struve, Herschel, Le Verrier, 
Smyth, Hind, Gauss, Airy, Rosse, Bond, Alexander, Maury, Gould, Olmstei > 
Peirce, and many others, who have contributed largely to astronomical science, 
"wUl ever be regarded among the brightest stars of the nineteenth century. 



NOTES. 



Divisibility of Matter. 

Note 1, Page 16. 

As far as our experience goes, we find that all bodies may be divided and sub- 
divided even beyond the limits of sensible perception. 

A single grain of gold may be divided into many millions of parts, visible by the 
aid of the microscope. The gold which covers the silver wire used in making 
gold lace is beaten out so that a single grain would cover a surface of nearly 
thirty square yards. In odoriferous bodies, such as musk, an apartment may be 
filled year after year with the most intensely penetrating odor, without any per- 
ceptible loss of weight. 

In cinnabar may be foimd another example of the divisibility of matter. It is 
composed of mercury and sulphur, and may easily be separated into these con- 
stituents ; yet, says Professor Miiller, under the best microscopes it appears to be 
a perfectly homogeneous mass, the small particles of sulphur and mercury being 
undistinguishable. 

But although divisibility extends far beyond the limits of our senses, it must 
not be inferred that it is wholly unlimited. Those minute particles of matter 
which cannot be further disintegrated are, by philosophers, termed atoms. 

Galileo's Experiment. 

Note 2, Page 18. 
Aristotle, more than two thousand years ago, taught, among other philo- 
sophical errors, that if two bodies of unequal weight were to be let fall from the 
same height, at the same moment, the heavier body would move as much swifter 
than the lighter as its weight exceeded that of the lighter body. Galileo, a great 
Italian philosopher and astronomer, discovered the fallacy of this theory, and, on 
subjecting it to experiment, proved beyond a doubt that the velocities of falling 
bodies do not depend upon their weight. As may be supposed, Galileo met with 
opposition from all sides in the promulgation of his new philosophy. His oppo- 
nents of the Aristotelian school accepted a challenge from him to submit their 
respective theories to the public test. The celebrated Leaning Tower of Pisa was 
chosen for the performance of the experiment. At the time appointed, each 
party repaired to the spot, equally confident of success. The experiment was 
made with two balls, the one precisely one-half the weight of the other. The fol- 
lowers of Aristotle asserted that the heavier ball would reach the earth in one- 
half the time it would require the lighter one to descend ; and Galileo maintained 
that the diflference of weight would not afi"ect their velocities, but that they would 

S4o 



346 bouvier's familiar astronomy. 

both reach the ground at the same moment. On the day appointed, in the pre- 
sence of a large concourse of people assembled to witness the performance, the 
experiment was made, and Galileo came off victor, both balls having touched the 
ground at the same instant. 

Gravitation, and Progress of its Discovery. 

Note 3, Page 18. 

There is no doubt that Plato had a clearer idea than Aristotle of the attractive 
force exercised by the Earth's centre on all matter removed from it. Aristotle, as 
■well as Hipparchus, was acquainted with the acceleration of falling bodies, with- 
out correctly comprehending the cause. Plato, as well as Democritus, conceived 
there was attraction between bodies whose elements were homogeneous. 

John Philopenus, the Alexandrian, a pupil of Ammonias, who was a philosopher 
of the eclectic school, flourished in the fifth or sixth century of our era. He was 
the first who ascribed the movements of the heavenly bodies to a primitive im- 
pulse ; connecting with this theory that of the fall of bodies, or the tendency of 
all substances, whether heavy or light, to reach the Earth. 

Kepler, in his work De Stella 3Iartis, observes that " two insulated bodies would 
move towards each other like two magnets, describing spaces reciprocally as their 
masses ; and that if the Earth and Moon were not held by some force at the dis- 
tance which separates them, they would come in contact. The Moon, being the 
smaller body, would describe || of the distance, and the Earth the remaining 3L, 
supposing them to be of equal density. If the Earth ceased to attract the waters 
of the ocean, they would be drawn to the Moon by the attractive force of that 
body. The attraction of the Moon, which extends to the Earth, is the cause of 
the ebb and flow of the sea." 

Thus De Stella Martis contains the germ of the theory of gravitation, which the 
genius of Newton developed. 

The idea expressed by Kepler, of the ebb and flow of the ocean being caused by 
the attractive influence of the Moon, received in 1666 and 1674 a fresh impulse 
and a more extended application through the sagacity of the ingenious Robert 
Hooke, a noted experimental philosopher, who distinguished himself by numerous 
discoveries in science. But Newton's theory of gravitation, which followed these 
earlier advances, presented the grand means of converting the whole of Physical 
Astronomy into a true mechanism of the heavens. This great philosopher ob- 
served that the force of gravitation on the summits of the highest mountains is 
nearly the same as at the level of the sea; from which he inferred that the 
attractive influence of the Earth extends to the Moon, and combining with her 
projectile force, retains her in her orbit around the Earth. Hence followed the 
inference that if terrestrial gravitation retains the Moon in her orbit, solar gravi- 
tation must be the principle which influences the planets, when combined with 
their projectile forces, to perform their revolutions round the Sun. This theory 
he proved to be true, by showing that the areas were proportional to the times. 

Notwithstanding the theory of gravitation was proved by Newton, it was attained 
by means of such intricate reasoning as soared far above the reach of ordinary 



NOTES. 347 

mindH ; consequently, a very small number of mathematicians were qualified to 
appreciate the evidence upon ■which the conclusions of Newton were founded, the 
methods adopted being almost exclusively the creation of his own genius. From 
this cause the doctrines of the "Principia" were long neglected, and afterwards 
violently opposed, particularly on the continent of Europe. Huyghens, notwith- 
standing his high admiration for the genius of Newton, was one of the strong oppo- 
nents of the theory of universal gravitation. Although he admitted the mutual 
gravitation of the planets and satellites, according to the law of the inverse square 
of the distance, he could not be persuaded to assent to the doctrine of the mutual 
attraction of the parts of which those bodies are composed. In one of his works, 
in allusion to Newton's theory of the figure of the Earth, he says it is inadmis- 
sible, inasmuch as it supposes that all particles of matter have an affinity for 
each other; which, he says, cannot be reconciled with the established laws of 
mechanics. 

In Germany, the great Leibnitz was a formidable opponent of the Newtonian 
theory. He embraced the opinions of Descartes with regard to the planetary 
motions. 

In France, Cassini and Maraldi were strenuously opposed to the new philoso- 
phy, and their example was generally followed by contemporary astronomers. It 
was not until about the middle of the eighteenth century that the theory of gravi- 
tation was confirmed beyond any doubt. 

The illustrious discoverer of this universal law claimed no particular talents 
above other inquirers into the secrets of nature. In a letter to Dr. Bentley, New- 
ton said — "If I have done the public any service in philosophical discoveries, it is 
due to nothing but industry and patient thought." 

As far as human knowledge extends, the intensity of gravitation has never 
varied within the limits of the solar system, nor does even analogy lead us to 
expect that it should ; on the contrary, there is every reason to believe that the 
great laws of the universe are immutable, like their Author. 

Centre of Gravity. 

Note 4, Page 20. 
The discovery of the centre of gravity is due to Archimedes; many skilful 
geometricians endeavored to discover its position, both in plane surfaces and 
solids. Euler, believing it to depend only on the figure of the body, called this 
point the centre of inertia; it may also be called the centre of parallel forces. 

Laws of Motion. 

Note 5, Page 22. 

A CANNON ball of a thousand ounces, moving one foot per second, has the same 
quantity or force of motion as a musket ball of one ounce, leaving the gun barrel 
with the velocity of a thousand feet in a second. 

From these general principles flow directly three axioms, called the laws of mo- 
Hon. These are termed Newton's Laws, but they first appeared in Descartes' s Prin- 
cipia Philosophioi, Part II. p. 38, a work which appeared before Newton's Frinctpia. 



348 bouvier's familiar astronomy. 

These three laws are as follows : — 

Law I. Every body perseveres in its state of rest, or uniform motion in a straight 
line, unless it is compelled to change that state by forces impressed thereon. 

When a body is at rest, it would remaia so forever, if it were not put in motion 
by some external force. If a cannon ball be shot in a certain direction, it would 
continue to move in a straight line, if it were not checked by the resistance of the 
air, and drawn to the earth by the force of gravity. 

Law II. The alteration of motion, or motion generated or destroyed in any body, 
is proportioned to the force applied; and is made in that straight line in which the 
force acts. 

If a certain motion be generated by a given force, a double motion would be 
produced by a double force, if both forces operate in the same direction. If a 
body be acted on by two equal forces in different directions, it will take a direc- 
tion differing from both, and intermediate between the two. But if it be acted on 
by two equal forces in opposite directions, the body will remain in a state of rest ; 
the two forces, being equal, will neutralize each other. 

Law III. To every action there is always opposed an equal reaction ; or the mu- 
tual actions of two bodies upon each other are always equal and directed to contrary 
points. (See Newton's Principia, Book I.) 

If there be a pound weight in one scale, we must use the force of a pound to 
raise the weight. 

Centrifugal Force. 

Note 6, Page 26. 

We have, in all circular motions, the example of a continued acting force. The 
motion of matter abandoned to itself would be uniform and rectilinear ; it is evi- 
dent, therefore, that a body moving in a circle has an increasing tendency to fly 
off from the centre at a tangent. In circular motion, the central or centripetal 
force is equal and directly contrary to the projectile or centrifugal force. The 
former attracts the body to the centre, and in a very small interval of time its 
effect may be measured by the versed sine of the arc described.^ 

As the centrifugal force is increased by increasing the velocity of a revolving 
body, if the Earth were to revolve more rapidly on her axis than she now does, 
the force of gravity would be diminished. 

The force of gravity at the equator is known to be 289 times greater than the 
centrifugal force ; and as 289 is the square of 17, it follows that if the Earth were 
to revolve 17 times faster than she now does — that is, to complete her revolution 
in 85 minutes instead of twenty-four hours — the centrifugal force would be en- 
tirely neutralized. In this case the centrifugal force would be equal to that of 
gravity, and bodies at the equator would have no weight. But suppose the Earth 
were to complete a diurnal revolution in a still less time than 85 minutes, all 
small and light substances would fly off from the surface ; and if the velocity were 
greatly increased, the more massive portions of the Earth's surface would fly off 
and rotate around it. 

* La Place, Sr/steme du Monde, Liv. III. chap. ii. 



NOTES. 349 

Angular Measurement. 

Note 7, Page 28. 

It is manifest that our sight can give us no other information regarding the 
bodies which are distributed in space around the Earth, than the facts of their 
apparent positions and dimensions. These can only be exactly determined by 
means of angular measurement ; that is, by noting the amount of divergence that 
separates the straight lines along which they are severally viewed. The astro- 
nomer first carefully and accurately ascertains the apparent positions of the 
heavenly bodies and their apparent dimensions in this way, and then from the 
apparent positions and dimensions which he has observed, he deduces their real 
sizes and distances, and also their true arrangements and motions in space. 

It must be borne in mind that apparent dimensions and positions are very differ- 
ent from real dimensions and positions. The former are merely the rough ele- 
ments by means of which the latter may be found through a complicated process. 
The apparent size of a body is the amount of divergence by which two straight 
lines are separated, coming to the eye from either of its extremities. In the same 
way the apparent positions of two bodies are merely the amount of divergence 
by which two straight lines are separated, coming to the eye from each of them. 

It will be understood that when a planet, the Sun, or Moon are said to appear 
so many degrees, minutes, or seconds from each other, those degrees, minutes, or 
seconds have always been merely the corresponding proportional parts of the 
great circle of the heavens used as standards of comparison. 

Atmosphere of the Sun, and Solar Spots. 

Note 8, Page 31. 

The resplendent nature of the Sun is now supposed to arise from a luminous 
atmosphere, or photosphere, as it has been termed, which is the source of light 
and the partial cause, at least, of heat, throughout the system. Of the precise 
nature of this envelope we are yet ignorant, but that it exists is almost beyond 
dispute, from the appearances revealed by the telescope. 

It is conjectured that there are three atmospheric strata about the sun : that 
supposed to lie nearest his surface is called the cloudy stratum, being of a charac- 
ter incapable of reflecting light, and heavily loaded with vapors ; the next in 
elevation is thought to consist of an intensely luminous medium, and to this is 
attributed the diffusion of light and heat ; at a greater altitude still, it is probable 
there exists a third envelope of a transparent gaseous nature. 

Sir John Herschel says, "Above the luminous surface of the Sun there are 
strong indications of the existence of a gaseous atmosphere, having a somewhat 
imperfect transparency. When the whole disc of the Sun is seen at once through 
a telescope magnifying moderately enough to allow it, and with a darkening 
glass, such as to suffer it to be contemplated with perfect comfort, it is very evi- 
dent that the borders of the disc are much less luminous than the centre. This 
can only arise from the circumferential rays having undergone the absorptive 
action of a much greater thickness of some imperfectly transparent envelope (due 
to the greater obliquity of their passage through it) than the central. But a still 



;50 



BOUVIER S FAMILIAR ASTRONOMY. 




more convincing, and indeed decisive evidence, is offered by the phenomena 
attending a total eclipse of the Sun, which prove the existence of an atmosphere 
as above described." HerscheVs Ast. p. 209. 

When viewed through the telescope, the Sun has the appearance of an enormous 
globe of fire, the surface of which appears mottled or grained, and frequently in 
agitation. Irregularly-shaped black spots are sometimes to be seen on his disc, 
grouped together, as in the following figure. 

Fig. 198. Spots of this kind have been known 

to be so large as to be distinguished 
by the naked eye. One is recorded 
as having been seen by Sir William 
Herschel in 1779, which was 30,000 
miles in diameter. The spots appear 
to move from east to west on the 
Sun's disc in about fourteen days. 
Dark spots are usually surrounded 
by a penumbra, and that again by a 
margin of light brighter than the 
other parts of the disc. When a spot 
is first seen on the eastern edge of 
the Sun, it appears like a line, which 
enlarges in breadth until it reaches 
the middle of the disc, when it begins 
to contract, and finally disappears on the western limb. After a space of fourteen 
days, they have been known to reappear on the east side. 

The spots often retain their form for several days without material change, and 
sometimes only a few hours are required to witness astonishingly rapid changes 
in them. A large spot has often been seen to separate, and, as it were, break 
into small -fragments, in the space of a few minutes. Small spots, too, sometimes 
coalesce and form one large one in a very short space of time. 

The paths which the spots describe about the beginning of June and December 
are observed to be rectilinear. But from the month of June to December, the 
lines described are convex to the north ; and from December to June, the line of 
spots is convex to the south. From these circumstances, it has been conjectured 
that the spots are attached to the surface of the Sun, and that the Sun rotates on 
his axis, which is inclined 7° 30^ to the axis of the ecliptic. 

Sir William Herschel supposed the Sun to be a solid, dark nucleus, surrounded 
by a vast atmosphere, almost always filled with luminous clouds, occasionally 
opening and discovering the dark mass within. The speculations of La Place 
were — that the solar orb is a mass of fire, and that the violent effervescences and 
ebullitions seen on its surface are occasioned by the eruption of elastic fluids 
formed in its interior ; and that the spots are enormous caverns, like the craters 
of our volcanoes. 

The light is more intense at the centre of the Sun's disc than at the edges, 
although, from his spheroidal figure, the edges exhibit a greater surface than the 
centre under the same angle. This fact is accounted for, on the supposition that 



NOTES. 351 

the Sun is sm rounded by a dense atmosphere, which absorbs a portion of the I'ays, 
which have to penetrate a greater extent of it at the edges than at the centre. 
The reverse is the case with the Moon, which is supposed to be devoid of an ap- 
preciable atmosphere, for the light at the edges of her disc is more brilliant than 
at the centre. 

Diameters of the Planets Measured. 

Note 9, Page 40. 

*' The apparent diameters of the Sun, Moon, and planets are determined by 
measurement ; therefore their real diameters may be compared with that of the 
Earth : for the real diameter of a planet is to the real diameter of the Earth, or 
7916 miles, as the apparent diameter of the planet to the apparent diameter of 
the Earth as seen from the planet: that is, to twice the parallax of the planet." 
Mrs. Somerville, Con. Phys. Set., p. 55. 

When the apparent size and real distance of a body are known, it is easy to cal- 
culate its true dimensions. 

Fig. 199. 



E — = 




Let a a represent the diameter of the Sun S, and E a E a its distance from the 
Earth. Then a E a is a triangle, of which two sides and one angle are known : 
the angle E being the apparent breadth of the Sun's disc, compared with the en- 
tire circle of the heavens, of which H H is an arc. The angular measurement 
of the Sun is 32^ 3'^, or 1923'^ out of the 1,296,000^^ which the entire circum- 
ference of the heavens contains. The sides E <z E a are each the Sun's distance ; 
that is, 95,000,000 of miles. But when two sides and one angle of a triangle are 
known, the third side can be calculated. Hence, the side a a, which is the Sun's 
diameter, has been found to be about 888,000 miles. The planets are measured 
in the same manner. 

Seasons of Venus. 

Note 10, Page 41. 
It is generally believed that the planet Venus is inclined to the plane of her 
orbit at an angle of 75°. It is well known that the inclination of the axis of the 
Earth is 23° 28'. Now, it is the inclination of the axis of a planet to the piano 
of its orbit which constitutes the annual viscissitudes known as the seasons. 



352 



BOUVIER S FAMILIAR ASTRONOMY. 



On the 21st of March the days and nights are equal all over the Earth, the Sun 
being then vertical at the equator. From that time till the 21st of June the 
days increase in length in the northern hemisphere. On the 21st of June, the 
Sun arrives at the Tropic of Cancer, his greatest northern declination, -which is 
23° 28'' north of the equator ; at \vhich time the days are longest in the northern 
hemisphere. He then seems to recede southward until the 21st of September, 
when he is vertical again at the equator ; at which time the days and nights ar«' 
equal aU over the Earth. From the 21st of September the Sun seems still tf 
recede farther south, until the 23d of December, when the days are shortest ir 
the northern hemisphere. Thus, it will be seen that the Sun is always vertical 
to some places in the torrid zone ; and that the boundaries of that zone, called the 
tropics, are his utmost northern and southern declinations. 

Fig. 200. 




Of the above engravings, fig. 200 represents the Earth, and fig. 201, the planet 
Venus. C C are the planes of their respective orbits, P P their poles, and E E 
their equators. P P represent their axes as well as their poles. By referring to 
the figures, it will be seen that the axis of Venus is much more inclined from the 
perpendicular of her orbit than that of the Earth. T T indicates the tropics in 



NOTES. 353 

the two planets. It will be remembered that the tropics are the utmost limits at 
which the Sun is vertical. Now, on the planet Venus the tropics being within 
162^° of her poles, it follows, that the Sun is at one time or other vertical to nearly 
all the points on her surface. As the tropics are the boundaries of the torrid 
zone, and as they are situated 73J° on each side of the equator, the torrid zone on 
the planet Venus must be 147° in width. As the tropics are within 15° of her 
poles, her polar circles must be within 16J° of her equator. 

By this arrangement, the Sun is vertical twice a year to all places on the planet 
Venus, except those situated within 16|° of each pole. During one-half of Venus's 
year — that is, sixteen weeks — the Sun continues at one pole without setting, while 
the inhabitants of the other pole are involved in darkness. In this respect Venus 
resembles our Earth, for each pole has a night of half a year. But, unlike the 
Earth, the inhabitants at Venus's equator have two winters and two summers in 
every year. Let it be remembered that Venus's year is only about thirty-two 
of our weeks. 

Form of the Earth. 

Note 11, Page 45. 

The form of the Earth, as well as that of the planets and satellites, is owing to 
the reciprocal attraction of their component particles. For instance, a fluid mass, 
of any size whatever, if in a state of rest, would assume a spherical form, owing 
to the reciprocal attraction of its component particles. But if it be made to re- 
volve on an axis, it at once becomes a spheroid, more or less flattened at the poles 
in proportion to the velocity of its axial rotation ; for the centrifugal force arising 
from this velocity diminishes the gravity of the particles at the equator, which 
recede from the centre till by their number and attraction they counterbalance 
the centrifugal force. 

The form of the Earth furnishes a standard of the weights and measures in 
ordinary use. The British measure of length is formed by noting the length of a 
pendulum vibrating seconds in the latitude of London. Its length was found by 
Captain Kater to be 39-1387 inches, when oscillating in vacuo at the temperature 
of 62° Fahrenheit, and reduced to the level of the sea. The weight of a cubic 
inch of distilled water, at the same temperature, and the barometer 30 inches, was 
also determined in parts of an imperial Troy pound ; from these data the British 
standard of weight and measure can always be known. 

The French have adopted the metre for their unit of linear measure, which is 
the ten-millionth part of that quadrant of the meridian passing through Formen- 
tera and Greenwich, the middle of which is nearly in the forty-fifth degree of 
latitude. 

The Ellipse. 

Note 12, Page 47. 
The ellipse, popularly called an oval, is one of the conic sections. The name 
of ellipse was given to it by Appolonius, among the ancients the first and prin- 
cipal writer on the conic sections. 

This figure is variously defined by diff'erent authors, either from some of its 

23 



354 



BOUYIER S FAMILIAR ASTRONOMY. 



properties, from its mechanical construction, or from the section of a cone, which 
is the best and most natural method. Thus : An ellipse is a plane figure made 
by cutting a cone obliquely, by a plane passing through its opposite sides. 

Fig. 202. 




In order to construct an ellipse, the simplest method is to take a loop of thread 
of the length of the longer axis, fasten two pins (which represent the focal points 
of the ellipse) through the paper on a board, and pass the loop of thread over 
them ; then place a pencil inside of the loop, stretch it with the pencil, and trace 
round the pins : the pencil will describe a curve called an ellipse. 

In the above figui-e let F / be the pins, and F C / E the loop of thread ; by 
stretching out the loop, as at E. with a pencil, the ellipse A E B D may be traced. 

The line A B is the longer or transverse axis, the perpendicular to which, D C E, 
is the conjugate axis. F/are the two focal points or foci of the ellipse, and C, 
the intersection of the transverse and conjugate axes, is the centre. 

The ellipse is a figure of such a nature that if two lines be drawn from two 
certain points in the axis to any point in the cii^cumference, the sum of these two 
lines will be everywhere equal to the transverse axis. 

If the two dotted lines F H/H be drawn from the points F/on the transverse 
axis to a point H in the circumference, the sum of these two lines will be found 
equal to the axis AB. The sum of the lines F E/E will also be found equal to 
the transverse axis A B, and also to the sum of the dotted lines F H/H. 



Kepler's Laws. 

XoTE 13, Page 48. 
A PLANET moves in its elliptical orbit with a velocity varying every instant, in 
consequence of two forces : the one tending to the centre of the Sun, and the 
other in the direction of a tangent to its orbit arising from the primitive impulse 

given at the time when it was launched into space. Should the force in the tan- 



NOTES. 



355 



gent cease, the planet would fall to the Sun by its gravity. Were the Sun not to 
attract ft, the planet would fly off in the tangent. Thus, when the planet is at 
the point of its orbit farthest from the Sun, his action overcomes the planet's 
velocity, and brings it towards him with such an accelerated motion that at last 
it overcomes the Sun's attraction and shooting past him, gradually decreases in 
velocity until it arrives at the most distant point, where the Sun's attraction again 
prevails. In this motion the radii vectores, or imaginary lines joining the centres 
of the Sun and the planets, pass over equal areas or spaces in equal times. 

In a circle the radii are all equal ; but in an ellipse (fiff. 45, page 48) the ra- 
dius vector S 6 is greater, and S a less, than all the others. The radii vectores 
S e S/are equal to P a or P 6, half the major axis b a, and consequently equal to 
the mean distance. A planet is at its mean distance from the Sun when in the 
points /or e. Thus it will be seen that a planet's mean distance from the Sun is 
equal to half the major axis of its orbit. If, therefore, a planet described a circle 
round the Sun at its mean distance, its motion and periodic time would be uni- 
form, because the planet would arrive at the extremities of the major axis at the 
same instant, and would have the same velocity, whether it moved in the circular 
or elliptical orbit, since the curves coincide in these points. 

Kepler, by a long series of observations, became convinced that the planetary 
orbits could not have the properties belonging to a circle, as had been conjectured 
by former astronomers. He selected the planet Mars as a subject for his investi- 
gations. He soon found that the predicted place of that planet was often far 
from the true one, which led him to seek for a new theory upon which to build 
his superstructure. He saw the fallacies of eccentrics and epicycles, and deter- 
mined to try the next most simple curve — viz. the oval or ellipse. He placed the 
Sun in the centre of the major axis, hoping to follow out the planet through his 
elliptical orbit. But although at first he was flushed with success, he began to 
find that his hypothesis must be rejected. Finally, he placed the Sun in one of 
the foci of the ellipse instead of the centre, and was gratified to find his labors 
crowned with success. His theory was correct : he followed the planet Mars in 
his whole revolution, always finding him in the predicted place. 



In Jiff. 203 the ellipse T T T, &c. re- 
presents the true orbit of the planet, 
and the circle M M M, &c. the imaffi- 
nary orbit in which the mean place is 
designated. If the planets revolved in 
circular oi'bits, they would have a uni- 
form motion ; whereas, their motion is 
known to be sometimes faster or slower 
than a circular motion would be. At 
the extremities of the major axis — 
that is, at perihelion and aphelion — the 
mean and true motion is the same ; for 
at these points the elliptical orbit and 
the imaginary circular one coincide. 

By reference to the figure it Avill be 



Fig. 203. 




356 bouvier's familiar astronomy. 

seen that from perihelion to aphelion the true places of the planet, which are 
designated by the plain lines diverging from the Sun and terminating at the circum- 
ference of the ellipse, are ahead or eastward of the mean places, which are desig- 
nated by dotted lines diverging from the centre of the circle, and terminating at the 
circumference. From aphelion to perihelion the true place is behind the mean one. 

The discovery of the elliptical orbit of Mars was soon followed by that of the 
other planets, and also of the Moon. Kepler, shortly after this, proclaimed to the 
world his "jfirst law" — viz. The planets revolve about the Sun in ellipses, having the 
Sun in one of the foci. 

As the planets were now known to move in elliptical orbits, and as their velo- 
cities varied in different parts of their orbits, it remained to discover the law 
which governs their movements. Kepler applied himself assiduously to the task, 
and was rewarded by discovering the law which regulates orbitual motions. This 
is called Kepler's " second great law." If a line be drawn from the centre of the Sun 
to any planet, this line, as it is carried forward by the planet, will sweep over equal 
areas in equal portions of time. 

After seventeen years of incessant labor, Kepler discovered his third law. As 
the relative mean distances and periodic times of the planets were ascertained, 
Kepler thought there were certain mysterious analogies in the laws of nature, and 
endeavored to discover if any relation existed between their periodic times and 
their distances. After much patient research, he discovered that the squares of the 
periodic times of the planets are as the cubes of their mean distances from the Sun; 
which is known as Kepler's "third law." 

The above laws of nature are now known as Kepler'' s laws. 

" The laws of nature signify the enunciations of the method or will of God. If 
those who look coldly on science knew better its aims, we should have less of the 
infidelity of the term law, and find fewer infidels or rejecters of that revelation 
which God has spread out before us. To him whose mind has become deeply im- 
bued with science, nature becomes a living expression, as full as is possible in 
finite language, of the perfection of the Supreme Architect, with whom to create 
has ever been to evolve beauty amid displays of wisdom and beneficence." — Pro- 
fessor Dana's Address before the American Association for the Advancement of Science, 
August, 1855. 

The Asteroids. 

Note 14, Page 57. 

The bodies forming the group situated between the orbits of Mars and Jupiter, 
difi'er, in many respects, very widely from the other planets of the solar system. 
They are almost all telescopic, being too minute to be distinguished by the naked 
eye. They bear a strong resemblance to stars, even when viewed with a good 
telescope ; which induced Sir William Herschel to give them the name of Asteroids. 

The immense space existing between the orbits of Mars and Jupiter caused 
Kepler to suspect the existence of a planet, which idea was afterwards revived by 
the German astronomers, who took active measures to ascertain the fact. About 
this time Professor Bode announced the discovery of his law, an account of which 
is given in Note 15. So zealous were the German astronomers of that day to 



NOTES. 357 

throw light on this interesting question, that in the year 1800, six of them assem- 
bled at Lilienthal, and established a society consisting of twenty-four practical 
observers, whose duty it was to examine critically every telescopic star of the 
zodiac, in order, if possible, to discover the unknown planet. 

On the first day of the following year, (1801,) their hopes were realized in the 
discovery of Ceres, by Piazzi, at Palermo. But Piazzi announced to the world 
that he had discovered a comet, the disc being but faintly determined, owing to 
the small diameter of the planet and its great distance from us. 

Bode, however, immediately suspected its true nature, and from the slender 
data given by Piazzi its orbit was computed by Olbers, Burckhardt, and Gauss. 
This coincided very nearly with the computations of Baron de Zach for the orbit 
of the unknown planet ; which, having been made by him from analogy in 1 785, 
sixteen years previous, serves to show the triumph of the theory. This planet 
was called Ceres. 

But what was the astonishment of these astronomers, when, in the following 
year, the discovery of another planet was announced by Dr. Olbers, of Bremen ! 
And how much greater was their surprise, to find that this newly-discovered body, 
which they called Pallas, could not be restrained within the bounds of the zodiac, 
and that its orbit and that of Ceres approached so near, at the intersection of 
their planes, as to occupy but a narrow zone at the nodes ! This fact led Dr. Ol- 
bers to suppose that they were the fragments of a larger body, which had been 
burst asunder by some great convulsion, and, if so, fragments of the body might 
yet be discovered. 

This theory was strengthened in 1804 by the discovery of another small body, 
which M. Harding, its discoverer, called Juno. So convinced was Dr. Olbers in 
the truth of his theory respecting the common origin of these small planets, that 
as the mutual intersection of Ceres, Pallas, and Juno occurred in Virgo and Cetus, 
the explosion must have taken place in one of those regions, and therefore other 
fragments of this large body might be found there. His expectations were 
realized; for, in 1807, he discovered another small planet in the constellation 
Virgo, which was named Vesta. 

After the discovery of this last-named planet in 1807, Dr. Olbers continued his 
search until the year 1816 ; when it was thought unnecessary to make any further 
observations in either of the above-named regions — namely, Virgo and Cetus — as 
no planetary body could have escaped the notice of observers. Therefore, the 
plan was relinquished for a time. 

About the year 1830, M. Hencke, an amateur Prussian astronomer, undertook 
a survey of that zone of the heavens comprised between 15° on each side of the 
equator. He made himself so thoroughly acquainted with every minute tele- 
scopic star, that he could readily detect any planetary body in its orbit among 
them. After fifteen years of untiring diligence and unremitted labor, he discovered 
another small planet, which, having requested Professor Encke to name, he called 
Astrea. This planet was discovered in 1845, and in two years after, (1847,) 
M. Hencke announced the discovery of another asteroid, for which he requested 
Professor Gauss to select a name. Hebe was soon decided upon as the designation 
of the newly-discovered stranger. 



358 bouvier's familiar astronomy. 

la little more than a month after Hebe was introduced to the astronomical 
world, the distinguished English astronomer, Mr. Hind, at the private observa- 
tory of Mr. Bishop, in Regent's Park, London, discovered the seventh asteroid, 
which vras named Iris. The same observer found another planet — the eighth 
in order of discovery — in about two months after he discovered Iris. This new 
planet was named Flora, by the request of Sir John Herschel. 

In the spring of 1848, Mr. Graham discovered another planet, at the private ob- 
servatory of Markree Castle, Ireland. The name selected for this little planet is 
Metis. 

A year had not elapsed, before another member of this group was announced, 
by the name of Hygeia. Its discoverer was M. Annibal de Gasparis, assistant 
astronomer at the Royal Observatory at Naples. 

In 1850, but a little more than a year after the discovery of Hygeia, M. de Gas- 
paris announced another member of the asteroid family, which he named Par- 
thenope, at the suggestion of Sir John Herschel, that name having formerly been 
given to the city of Naples. 

Four months after the introduction to Parthenope, astronomers were made 
acquainted with the existence of Clio, or Victoria, as she is sometimes called. 
This planet was discovered by Mr. Hind, and is the twelfth in order of discovery. 

Only about seven weeks elapsed after the discovery of the last-named planet, 
till another was announced by M. de Gasparis, of Naples. This newly-discovered 
body was named Egeria, by M. Le Verrier. 

In May, 1851, Mr. Hind again proclaimed to the world the existence of a 
newly-discovered asteroid, which Sir John Herschel named Irene. 

About two months after, Eunomia was recorded by its discoverer, De Gasparis, 
as belonging to the group of small planets. This is the fifteenth in order of dis- 
covery. 

The sixteenth of the asteroidal group is called Psyche, and was also discovered 
by De Gasparis, of Naples, in the year 1852. 

M. Luther, the director of the observatory at Bilk, announced the discovery 
of the seventeenth asteroid, in 1852, one month after the discovery of Psyche. 
It received the name of Thetis. The year 1852 is noted for the discovery of no 
less than eight new planets, four of which were discovered by Mr. Hind. In June 
of that year, he discovered the eighteenth asteroid called Melpomene ; and in the 
same year, two months after, he discovered Fortuna, the nineteenth. On the 
19th of September of the same year, M. de Gasparis discovered Jlassilia, the twen- 
tieth ; and in the following November, M. Goldschmidt announced the discovery 
of Lutetia, the twenty-first asteroid. On the following day, November 16th, Mr. 
Hind discovered Calliope, the twenty-second ; and in December of the same year, 
the same astronomer discovered Thalia, the twenty-third of the group. 

In 1853, four new asteroids were added to the number already known — namely, 
Themis, the twenty-fourth, discovered by De Gaspai'is, April 5 ; and Phocea, the 
twenty-fifth, discovered by Chacornac on the following day, April 6. In May of 
that year, Luther discovered Proserpina, the twenty-sixth ; and in November, Hind 
discovered the twenty-seventh, which is called Euterpe. 

In the year 1854, six new planets were made known : the twenty-eighth, Bel- 



NOTES. 359 

lona, was discovered by Luther on the 1st of March ; Amphitrite, the twenty-ninth, 
was discovered on the following day, March 2, by Marth ; Urania, the thirtieth, 
by Hind, in July ; Euphrosyne, the thirty-first, in September, by Ferguson, at the 
Washington Observatory ; Pomona, the thirty-second, by Goldschmidt, on the 26th 
of October ; and the thirty-third by Chacornac, on the 28th of the same month. 
This is called Polymnia. 

M. Chacornac, of the Paris Observatory, discovered a new planet on the 6th of 
April, 1855, which was named Circe. It forms the thirty-fourth asteroid of 
the group. The thirty-fifth was detected by Dr. Luther, of Bilk, on the 19th of 
April of the same year, which was called Leucothea. 

This group of small planets is situated between the planets of small mass, which 
rotate slowly, and the greater bodies of our system, whose diurnal revolution is 
performed in a much shorter time. 

Whatever may be their origin, a subject on which astronomers differ so widely, 
they certainly are very dissimilar to the other components of the solar system. 
Their periods vary from a little more than three to five and a half years. Their 
orbits are all within a zone not exceeding one hundred millions of miles ; and 
their diameters, although not satisfactorily ascertained, are not supposed to ex- 
ceed three or four hundred miles at most. Some of them are believed to be even 
less than one hundred miles in diameter. The aggregate mass of these small 
bodies does not exceed, according to M. Le Verrier, the one-fourth of the mass of 
our Earth. 

Sir John Herschel says: "A man placed on one of them would spring with 
ease 60 feet high, and sustain no greater shock in his descent than he does on the 
Earth from leaping a yard." This is owing to the force of gravity being so much 
less on these small bodies. 

In reference to the theory of Dr. Olbers, Sir John Herschel remarks that ''"What- 
ever may be thought of such a speculation as a physical hypothesis, this conclu- 
sion has been verified to a considerable extent, as a matter of fact, by subsequent 
discovei-y." 

Mr, Hind thus concludes his remarks upon the theory of Olbers: "We have 
already alluded to the near approximation of the orbits of the small planets 
at the points of mutual intersection ; a circumstance which induced Olbers 
and many other astronomers to consider these bodies as the fragments of a 
large planet formerly revolving at about the same mean distance from the 
Sun, which had been shivered into pieces by some great internal explosion 
or an external shock. The idea of the German astronomer has been so 
strongly countenanced by the discoveries of the last five years, that we cannot 
fairly reject it, until another theory has been advanced which would account 
equally well for the peculiarities observed in the zone of planets, however un- 
willing we may be to admit the possibility of such tremendous catastrophes, 
and notwithstanding the great difference in the mean distances of Flora and 
Hygeia. Yet it may be found that these small bodies, so far from being por- 
tions of the wreck of a great planet, were created in their present state for 
some wise purpose, which the progress of astronomy in future ages may event- 
ually unfold." 



360 bouvier's familiar astronomy. 

On the other hand, M. Le Verrier is of opinion that the eccentricities and in- 
clinations of the orbits of the asteroids are entirely incompatible with the theory 
of Olbers ; and supposing them to have had a common origin, the force required 
to launch the fragments of the larger planet into the orbits which they now oc- 
cupy would be greater than we could, by any mathematical reasoning, be war- 
ranted in believing could ever have been exerted. His opinion is, therefore, that 
the asteroids are, and always have been, bodies smaller than the other planets ; 
and that they revolve in orbits suited to the projectile force which they received 
from the hand of the Creator. 

At a meeting of the "American Association for the Advancement of Science," 
which met at Providence, August, 1855, Professor S. Alexander communicated 
the results of his investigations on the probable origin of the asteroids. He con- 
siders it almost a certainty that in the space between Mars and Jupiter a planet 
once revolved, at about 268,000,000 of miles from the Sun, the equatorial diameter 
of which was about 70,000, and the polar diameter only about 8 miles. This 
body, therefore, was not a globe, but a disc. He supposed it to have been 
broken in fragments, owing to its velocity of rotation, as a grindstone will some- 
times burst when driven furiously. As the outer parts of this disc necessarily 
moved faster than those near the centre, they must have been thrown into larger 
orbits than those which move more slowly. The fragments near the circum- 
ference would have a larger orbit than the parent planet, and those near the 
centre would move in smaller orbits than their original. To those which moved 
most rapidly, and which now occupy the larger orbits, the point of explosion 
would be their perihelion ; while to those fragments which constituted the centre 
of the original planet, the place of explosion would be their aphelion, as their 
velocity would be diminishing. This theory of Professor Alexander is widely at 
variance with the views of many astronomers. 

Bode's Law. 

Note 15, Page 57. 

Between the orbits of Mars and Jupiter, there occurs an interval of about 
850,000,000 of miles, in which no planet was known to exist previous to the com- 
mencement of the nineteenth century. 

Kepler, who lived about 300 years ago, first entertained the opinion that a 
planet existed between the orbits of the planets Mars and Jupiter. But owing to 
certain superstitious notions which he, among other scientific men of those times, 
entertained with regard to numbers, particularly to the number seven, he re- 
nounced this idea for a time, believing it to be erroneous. Among some of the 
reasons assigned for the relinquishment of this theory is, that there are seven 
openings in the head — namely, two ears, two eyes, two nostrils, and one mouth ; 
and, also, that there are but seven tones in the gamut. 

But as he was an observing philosopher, as well as a mathematician, he was 
forced to return to his former opinion ; and in the first work which he ever pub- 
lished, he suggested that there must be a planet in the great void between Mars 
and Jupiter, or otherwise the harmony of the system would be broken. This 



NOTES. 361 

idea found favor in the German school, who, at a later period, embraced the 
theory, and resolved to test the truth of it. Their interest was aroused by the 
discovery of Uranus, by Sir William Herschel, in 1781. Although that planet 
was then the known boundary of our solar system, it served to account for cer- 
tain disturbances in the motions of Jupiter and Saturn which had never been 
explained. 

Professor Bode, of Berlin, embraced the opinion, first promulgated by Kepler, 
that an undiscovered body existed between the orbits of Mars and Jupiter, be- 
cause as we recede from the Sun the distances of the planets form a geometrical 
series of which the common ratio is 2, as will be seen by the following table, 
(allowing the distance of the Earth to equal 10,) which, however, is a mere ap- 
proximation to the truth, this theory not being considered by astronomers as suf- 
ficiently exact to deserve the name of a law : — 



Jupiter == 52 = 4 -f (3 X 24) 
Saturn = 100 = 4 -f (3 X 2^) 
Uranus = 196 = 4 -f (3 X 2^) 
Neptune = 388 = 4 + (3 X 2^) 



Mercury = 4 = 4 
Venus = 7 = 4 X (3 X 2") 
Earth = 10 = 4 -f (3 X 2') 
Mars = 16 = 4 + (3 X 22) 
Asteroids == 28 = 4 + (3 X 23) 

It will be seen, however, that this supposed law does not give the true distance 
for the planet Neptune. 

Neptune. 

Note 16, Page 64. 

The circumstances which attended the discovery of the planet Neptune were of 
a very extraordinary character. Uranus had often been observed by Flamstead, 
Le Monnier, and others, prior to its discovery by Sir William Herschel, and was 
each time recorded as a fixed star. It was thought to be the utmost boundary 
of the solar system ; and in order to ascertain its orbit, the most remote as well 
as modern observations were consulted. The first recorded notice of Uranus as a 
fixed star was in the year 1690 ; since then several places have been assigned to 
it, until the year 1781, when Sir William Herschel pronounced it to be a planet. 
From that time till the year 1820 its movements were narrowly and carefully 
watched. In 1821, M. Bouvard, of Paris, undertook to compare the entire series 
of observations, and from them to compute its place for any given time, and also 
to determine the exact nature of the ellipse in which it moves. But he found it 
impossible to reconcile the unlooked-for discrepancies which continually appeared, 
and to combine all the observations so as to form an elliptical orbit. He was there- 
fore induced to reject the earlier observations as unworthy of reliance ; and from 
the recent ones he traced out a path for Uranus, and noted the places in the 
heavens which he conceived the planet would occupy during the few following 
years. 

But fresh difficulties presented themselves ; for the computed places of the 
planet did not coincide with the true places, and this variation appeared to be on 
the increase. 

At this time. Dr. F. W. Bessel, a Prussian astronomer, publicly avowed that 



362 



BOUVIER S FAMILIAR ASTRONOMY. 



either there must be another unknown planet which produced these perturba- 
tions, or the law of gravitation cannot be universal. Observers found, however, 
that up to the year 1830, the planet never appeared in its predicted place ; for 
till then it was always in advance of Bouvard's calculation. It therefore became 
a matter of considerable interest to determine some plausible reason for this dis- 
agreement of theory and observation. It was strongly suspected that some un- 
known body might be attracting Uranus in some unascertained direction ; and it 
was demonstrated that if the attraction of an unknown body caused the disturb- 
ance, that body must be situated somewhere beyond the orbit of Uranus; for 
otherwise its influence would have been perceptible in the movements of Saturn. 

Two mathematicians in particular now devoted themselves to the task of at- 
tempting to find the situation of this disturbing body. These were M. Le Verrier, 
in France, and Mr. Adams, in England. M. Le Verrier sent his calculations to 
Dr. Galle, of Berlin, indicating the positions which the planet would occupy 
on certain given days. On the night following the day on which he received 
this communication — namely, the 23d of September, 1846 — Dr. Galle pointed his 
telescope to the designated spot, within a degree of which he detected the looked- 
for planet, in consequence of its presenting a large planetary disc under high 
magnifying power. At that very time Professor Challis was searching for it in 
the same field, under the direction of Mr. Adams ; and he subsequently found 
that he had really seen the planet seven weeks before Dr. Galle, and had entered 
its place twice in his list of observations. 

A reference to fig. 204 will make it at once 
plain why Neptune's power to disturb Uranus 
was so much greater just previous to the pe- 
riod of its discovery than it had been during 
the preceding century. Let S represent the 
Sun, U Uranus moving in its orbit round the 
Sun, and N Neptune travelling in his orbit 
beyond. Uranus performs his journey round 
the Sun in his smaller orbit in 84 years, 
whereas Neptune requires 164 years to per- 
form his orbitual revolution; consequently, 
if the planets started together from the posi- 
tions U and N, after 84 years Uranus would 
be again at U, but Neptune would then be 
only half round his orbit at n, and therefore 
the two bodies would be widely asunder. After another 84 years, Uranus would 
be at U for the third time, and Neptune would again be at N, so that the two bodies 
would be once more comparatively near together. They were in this relation shortly 
before the year 1830 ; Uranus had recently passed between Neptune and the Sun, 
and all the time it had been approaching Neptune it was attracted forward by the 
influence of the latter. In 1830, Uranus had already got so far past Neptune that 
this latter body now influenced it by holding it back, rather than by attracting it 
forward; and consequently, in 1830, Uranus was again found to occupy the place 
indicated by the tables of M. Bauvard. Thus was the existence of this planet 




NOTES. 363 

predicted, and its elements assigned, from considerations purely theoretical, show- 
ing the utility of mathematical knowledge in the study of the universe. 

After the discovery of Neptune, it became highly important to ascertain if any 
recorded observation of the planet could be found in the various catalogues of 
stars ; for should it be discovered that this body had been included in any 
catalogue, and its place recorded as a fixed star, a comparison between that 
position and the place it occupies at present, noting the interval of time between 
the two observations, would afford means whereby to compute its rate of motion, 
its periodic time, and other particulars dependent on these elements. 

After much research, it was found that this body had been observed and re- 
corded twice by Lalande. With these data our late distinguished countryman, 
Mr. Sears C. Walker, at that time connected with the Washington Observatory, un- 
dertook the investigation of the orbit of the new planet. He found that the period 
assigned by Le Verrier and Adams was more than 50 years too great, and that it is 
700,000,000 of miles nearer to the Sun than those astronomers had supposed. 
These elements, thus corrected by Mr. Walker, agree well with the calculations 
of Professor Peirce, and are considered by all astronomers as the most exact de- 
termination of the orbit of Neptune which has ever appeared. 

From the planet Neptune our Earth cannot be visible, even with the best tele- 
scopes, as it must always be oscillating, as it were, so immediately in the vicinity 
of the Sun, as to be entirely invisible. Even our transits could not be seen at that 
distance, owing to the inferior magnitude of our planet. 

Our Earth, as seen from the Moon. 

Note 17, Page 67. 

As our Earth is the primary centre of motion of the Moon, the lunar astro- 
nomer would at first be likely to suppose it stationary with regard to the other 
heavenly bodies, which to him appear to move slowly beside and behind it ; his 
own habitation, the Moon, seeming to be at rest also. 

Although the Earth would appear to occupy an almost fixed place in his firma- 
ment, its diurnal rotation would serve him as a perfect chronometer, no motion 
with which we are acquainted being so equable as that of the Earth on its axis. 
Our seas, mountains, plains, volcanoes, and regions of perpetual snow, by the con- 
trast of their reflected light, producing various shades of color, would be succes- 
sively brought into the view of an observer on the Moon by the rotation of the 
Earth on its axis. 

An inhabitant of the Moon will never see the Earth, if he should live on that 
hemisphere which is unknown to us; therefore, in order to enjoy mooiilight, he 
must travel round to that side of his globe which is always turned towards the 
Earth ; for our Earth is a moon to the lunarians. 

As the Moon possesses little or no atmosphere, the disappearance of the Sun is 
accompanied by the blackest night, except where the sunlight reflected from the 
Earth dispels the extreme darkness. 

When the Moon is in opposition to the Sun, ov full, the inhabitants of the Moon 
have new moon ; for our Earth is then between them and the Sun, appearing first 



364 bouvier's familiar astronomy. 

like a fine crescent, increasing continually, till finally, at the time of our new moon, 
our Earth would be opposite to the Sun, and consequently shine upon them with 
a full orb. The Moon is then new to us, and the light which the Earth reflects 
back upon the Moon, when only a small crescent of her illuminated hemisphere is 
towards us, enables us to see the outline of the dark part of the Moon's disc. 
This reflected earthlight is called by the French lumilre cendree; and by the vul- 
gar it is said to be the old moon in ike new moon^s arms. 

The mean apparent diameter of the Moon, as seen from the Earth, is SV 1"\ 
and the diameter of the Earth, as seen from the Moon, subtends an angle of 
1° 54''.^ By squaring these two apparent diameters, we find the proportional 
difi'erence between the surfaces is as 13 to 1, nearly. According to this calcula- 
tion, the Earth appears 13 times larger to the inhabitants of the Moon than the 
Moon appears to us ; and supposing our Earth to be as capable of reflecting light 
as the Moon, the lunarians must enjoy a moonlight thirteen times brighter 
than ours. 

As the Moon revolves upon her axis in the same time she performs a synodic 
revolution, which is in about 29J of our days, a day and night on the Moon must 
be equal to 29J days on the Earth. During this long continuance of sunlight, 
which would equal 15 times 24 hours, or half a lunar day, the heat would be ex- 
cessive ; and the absence of the Sun for as many successive hours would render 
the temperature below freezing. Thus each lunar day is an oppressive summer, 
and each night a rigorous winter. In one of our years the Sun rises and sets to 
the inhabitants of the Moon only about thirteen times. 

To an inhabitant of the middle of that hemisphere of the Moon which is turned 
towards us, the Earth will always appear on or near his meridian ; at the time 
of sunrise to him, the Earth will present half of her illuminated disc, the other 
half will be visible at his sunset. At his midnight the Earth will be/wZ/, giving 
as much light as thirteen full moons. 

Heat of Moonlight, 

Note 18, Page 77. 

That hemisphere of the Moon towards the Earth must necessarily be very 
much heated ; yet, until recently, no efi'ect has been perceived by the most deli- 
cate instruments. And although a slight increase of temperature may have been 
detected by some, the fact is not considered to be proved beyond a doubt. 

Lunar light was experimented on with a large burning-glass by the younger 
De la Hire, a French astronomer, at a time when the Moon was full and on the 
meridian. No sensible increase of heat could be discovered ; although the Moon's 
rays were concentrated so as to fill only the three-hundredth part of the space 
they naturally occupy, thereby possessing the power of three hundred full moons, 
yet no perceptible heat could be detected. 

Forbes, in the transactions of the Royal Society of Edinburgh, vol. xiii., gives it 
as his opinion, after many experiments, that moonlight is devoid of heat sufficient 
to be detected by the most delicate instruments known to us. 

Sir John Herschel is of opinion that the Moon's rays may possess heat, but 
that it may be absorbed in the upper regions of our atmosphere, which he thinks 



NOTES. 



365 



" the tendency to disappearance of clouds under the full moon" serves as a proof. 
But, on the other hand, Professor Loomis, in a paper read before the American 
Association, at Cleveland, Ohio, furnishes a table founded on the meteorological 
observations recorded at the Greenwich Observatory, -which serves to show that 
the Moon exerts no influence whatever on our atmosphere, and that our sky is as 
cloudy Sit full moon as at new. 

Observations have also been made at various times at Padua, Vienna, London, 
Munich, &c., by Toaldo, Pilgrim, Horsley, and Schiibler, the results of which tend 
to confirm the opinion that the Moon has no influence over our weather, which 
would undoubtedly be the case could any heat be detected in moonlight. 

The ancients attributed changes of weather to the Moon ; for we find the 
opinion entertained by some of the Greek writers, who no doubt received it, with 
their general notions of physics, from the nations of the East. They also believed 
not only the Sun, but some of the fixed stars, to have an influence on the weather. 
Pliny, a philosopher who lived in the first century of the Christian era, quotes 
Varro, (a Roman statesman and philosopher, who was born b. c. 116, and was the 
most learned man of his age,) who says : "If the upper horn of the Moon be ob- 
scure, the decline of the Moon will bring rain ; if the lower horn be indistinct, 
the rain will happen before full moon ; but rain may be expected at the time of 
full moon if the blackness be in the middle." — Plin. Nat. Hist. lib. xviii. cap. 85. 

Virgil makes the prognostic of the fourth day of the Moon decisive for the 
whole lunation. — Georgic, lib. i. lin. 143. 

In a letter to M. Arago, in the " Comptes Eendus," Melloni states that he has 
succeeded in detecting a raised temperature in moonlight, which had been con- 
densed by means of a lens three feet in diameter. Professor Mosotti-Lavagna, 
of the University of Pisa, and Professor Belli, of the University of Pavia, wit- 
nessed these "experiments of Melloni, which they considered satisfactory proof of 
the heat of moonlight. The amount of the deviations o^ temperature accorded 
with the age of the Moon and her altitude at the time of each observation. 

From the foregoing statements on both sides of the question, it will be seen that 
philosophers are still in doubt as to the heat of moonlight. 



Irregularities of the Moon's Motions. 

Note 19, Page 78. 
Fig. 205. 

2 




Let S (Jiff. 205) be the position of the Sun, E the position of the Earth, and n 
the Moon travelling round the Earth in its orbit. It will be evident that when 



366 bouvier's familiar astronomy. 

the Moon is at n the Sun attracts it more strongly than it does the Earth ; hence, 
at new moon the Sun's influence draws it away from the Earth. When the Moon 
is at/, the Sun attracts the Earth more strongly than the Moon; thus at full moon 
the Sun's influence tends again to increase the distance between the Earth and 
Moon. When the Moon is at 2, the Sun attracts the Moon in the direction S 2, 
but attracts the Earth in the line S n ; a similar action takes place when the Moon 
is at 1. Hence, towards the quarters of the Moon the Sun's influence brings the 
Earth and Moon continually a little nearer together than they would be without 
it ; for if two bodies move along two sides of a triangle towards its apex, they 
must necessarily approach each other. 

Again, when the Moon is moving from 1 to n, its motion is quickened by the 
Sun's attraction, because its path lies somewhat towards that body ; but when it 
is moving from n to 2, its velocity is retarded by the Sun's influence, because its 
path is receding from him. 

Among the numerous inequalities to which the Moon's motions are liable, the 
most prominent are the Equation of the Centre, the Evection, the Variation, and 
the Annual Equation. 

The Equation of the Centre is the difi"erence between the Moon's true and mean 
longitude, which vanishes at the apsides or extremities of the major axis, and 
attains its maximum at 90° distant from these points, or at the quadratures, 
where it is equal to the eccentricity of the orbit. The Equation of the Centre is 
equal to the angle formed at the planet, and subtended by the eccentricity of its 
orbit. 

The Evection is a variation in the equation of the centre depending on the posi- 
tion of the apsides of the lunar orbit. It was discovered in the first century by 
Ptolemy, but it remained for Newton to explain its true cause. When the lunar 
apsides are in syzigies, the action of the Sun increases the eccentricity of the 
Moon's orbit, or the equation of the centre. If the Moon be between the Earth 
and the Sun, or directly opposite to him, her distance from the Earth would be 
increased ; thus, at new moon the Sun attracts the Moon more than the Earth, 
and at full moon he attracts the Earth more than the Moon ; in either case aug- 
menting the Moon's distance from the Earth, and increasing the eccentricity, or 
equation of the centre. This increase is the evection, which may be more fully 
explained by the following figure. 

Fig. 206. 



Q 




If the Moon be in conjunction, as at A, the Sun attracts her from the Earth E ; 
and if the Moon be at the point P, he attracts the Earth more than the Moon ; 
consequently, the lunar orbit is rendered more elliptical, whicli increases the 
equation of the centre. The Moon's gravitation to the Earth is at its maximum 



NOTES. 



367 



when the Moon is 90° from the Sun, or in quadratures, which tends to diminish 
the distance Q E. 

If the line of apsides be at right angles to the radius vector of the Earth, the 
attraction of the Sun increases the distance of the Moon from the Earth, by which 
means it diminishes the eccentricity of the Moon's orbit, and causes it to approach 
nearer to a circle. If the Moon be in quadratures, the increase in her gravitation 
lessens her distance from the Earth, which also diminishes the eccentricity, and 
thereby the equation of the centre. 

Fig. 207. 





Let A P be the line of apsides, and S E the radius vector of the Earth ; when 
the Moon is at M or 0, the attraction of the Sun tends to increase the distance of 
the Moon from the Earth, and consequently to bring the lunar orbit more nearly 
to a circle. The force of gravity when the Moon is at A or P in quadratures 
would have the same tendency ; that is, to render the lunar orbit less elliptical, 
and consequently to diminish the equation of the centre. This diminution is the 
evection. 

The Variation is an inequality in the Moon's longitude, by which the Moon's 
velocity is accelerated, but at an unequal rate ; its maximum happening in the 
octants, or midway between the quadratures and syzigies, the acceleration being 
zero at the quadratures and conjunctions. 

Fig. 208. 

T 




The Sun's force, acting on the Moon in the direction S M, may be resolved into 
two other forces — one in the direction M E, which produces tlie evection, the 
other in the direction M T, tangent to the Moon's orbit. This tangential force 
produces the variation, by accelerating the Moon's velocity. 



868 bouvier's familiar astronomy. 

Variation was discovered by Tycho Brahe, but was explained by Newton. 

The fourth lunar inequality is the Annual Equation, which arises from a varia- 
tion in the distance of the Earth from the Sun, and is consequently owing to the 
eccentricity of the terrestrial orbit. When the Earth is in perihelion, the action 
of the Sun is greatest, which thereby dilates the lunar orbit, so that the angular 
motion of the* Moon is diminished ; but as the Earth approaches aphelion, her 
orbit contracts, and the Moon's angular motion is accelerated. 

Annual Equation was discovered by Tycho Brah^, by computing the places of 
the Moon for the various seasons of the year. He thus observed the real motion 
to be slower than the mean motion for six months from perihelion to aphelion, 
and faster during the other six months. 

Has the Moon an Atmosphere ? 

Note 20, Page 78. 

Whenever the light of the heavenly bodies passes through a dense atmosphere, 
they do not appear in their true places ; on this account, the Sun is seen from the 
Earth's surface when he is really below the horizon. If the Moon had an atmo- 
sphere of any appreciable density, the apparent position of a star would be 
changed during an occultation, so long as it was viewed through the denser me- 
dium. In other words, the star would be visible through the Moon's atmosphere 
when it was really behind the Moon, just as the Sun is seen through the Earth's at- 
mosphere when he is below the horizon ; in that case, the star would be concealed 
a shorter time than it ought to be by a body of the size of the Moon. 

If the Moon had an atmosphere possessing only the 1980th part of the density 
of the Earth's atmosphere, this influence would be discernible ; but no such result 
takes place, and therefore it is known that the Moon has not an atmosphere of 
even this trifling density. If our satellite is devoid of this gaseous envelope, the 
heavenly bodies must appear to be placed on a perfectly black background ; there 
could be no vapors nor rain, clouds nor air, and consequently no sound ; neither 
animal nor vegetable life, such as is common to our Earth, can exist there ; naught 
but a barren and voiceless wilderness, subject to the extremes of heat and cold. 
But these are only our suppositions ; He who created the lunar orb has fitted it as 
an agreeable abode for the intelligences he may have been pleased to place there. 

In June, 1831, Captain Smyth observed, under peculiarly favorable circum- 
stances, a passage of the Moon over the planet Jupiter. The illuminated half of 
the Moon first passed over the planet, and then the dark half; so that the planet 
was seen to emerge from behind the unilluminated part of the Moon's disc. As the 
planet appeared from behind the Moon it was singularly distinct and clear, the 
markings on its surface were evenly and boldly cut by the Moon's body ; neither 
its color nor the brilliancy of its light were in any way aff'ected. From these ob- 
servations. Captain Smyth concluded that the Moon cannot have an atmosphere of 
any appreciable density. 

In the dark part of the new moon a luminous spot has been frequently observed 
by Cassini, Sir William Herschel, Captain Kater, and Baily. In 1794, the same 
phenomenon was seen by two persons with the naked eye. This luminous appear- 



NOTES. 



369 



ance is described by Dr. Maskelyne in vol. Ixxxiv. of the Philosophical Transac- 
tions. It appears in the vicinity of the mountain known as Aristarchus, the out- 
line of which may be distinctly seen, although not illuminated by the Sun's rays. 
In the centre of this mountain a bright spot resembling a star of the ninth or 
tenth magnitude may sometimes be seen for several seconds, shining quite bril- 
liantly. Should this light result from the action of volcanoes, there would be no 
longer a doubt that the Moon is surrounded by an atmosphere, for fire cannot be 
maintained without air. Smyth, in his Celestial Cycle, vol. i. p. 133, says: "I 
have myself occasionally, though rarely, observed both a diminution of a star's 
brightness and an apparent projection of a star upon the Moon's disc at the in- 
stant of contact." These facts serve as proofs of the existence of a lunar atmo- 
sphere, yet the question is an unsettled one ; but if our satellite is surrounded by 
an atmosphere, it must be of a nature totally different from our own. 

At a meeting of the American Association for the Advancement of Science, held 
at Providence, R. I., August, 1855, there was a diversity of opinion with regard to 
a lunar atmosphere. Dr. B. A. Gould, Jr., called attention to observations made 
by himself and Professor Winlock, tending to establish the existence of a twilight 
at the Moon. This phenomenon was particularly observed at the time of new 
moon. The twilight appeared to be distinctly defined ; its breadth, however, did 
not measure more than two seconds. 

Eclipses of Jupiter's Satellites. 

Note 21, Page 89. 

Early in the seventeenth century the important discovery of the system of 
Jupiter furnished an undeniable proof of the truth of the Copernican theory, and 
afforded satisfactory evidence of the principle of universal gravitation. 

The eclipses of Jupiter's satellites are of frequent occurrence, and may be seen 
by means of telescopes of very moderate power. The shadow of the enormous 
sphere of that planet extends out into space more than half the distance of the Sun 
from the Earth ; that is, about 56,000,000 of miles. The three nearest satellites 
suffer eclipse every time they pass behind the planet ; the fourth sometimes escapes 
eclipse, because, like our Moon, its orbit is inclined to the orbit of its primary, 
so that its path at times lies above or below the edge of the shadow. 

Fig. 209. 




In the figure let S represent the centre of the Sun and J the planet Jupiter. 
The cone o F m will represent the shadow of Jupiter, and the spaces C o F and 

24 



370 



BOUVIER S FAMILIAR ASTRO>'OMY. 



F TO D vrill be the penumbra, from which a part of the Sun's light will be excluded. 
Thus it is very diflBcult to note the precise time of the disappearance of a satellite, 
as it partially disappears before it actually enters the shadow, its light being 
much obscured by the penumbra. Hence it will be seen that the disc of a satel- 
lite may become invisible to us before it is totally eclipsed. The brilliancy of the 
planet itself also prevents us from detecting the immersion with precision. 

In the foregoing figure, e h represents the portion of its orbit which a satellite 
must describe while passing through the planet's shadow, and Ip the section of its 
orbit which lies in the penumbra. 

When the satellite is moving from west to east, or in the direction of the arrow 
with regard to an observer on the Earth, it is liable to eclipse ; but when it ap- 
pears to be moving from east to west, it is then seen to transit the planet's disc 
like a bead of light ; while on other occasions the satellites have been observed as 
dark spots ; which is a very singular phenomenon, when we consider that their illu- 
minated hemispheres are then turned towards us. The only solution of this diflS- 
culty seems to be that some of the satellites are known to vary in brilliancy, owing, 
it is supposed, to spots on their surfaces less adapted for reflecting light than the 
other portions of their discs. 

Before opposition, the immersions of the satellites into the planet's shadow are 
visible, the emersions being usually hidden by the body of the planet. 



Fig. 210. 




In the above figure let S be the Sun and D E F an arc of the Earth's orbit ; h e 
a portion of the concave sphere of the heavens, between which and the orbit of the 
Earth is the planet Jupiter, casting his conical shadow opposite to the Sun. Let 
the Earth, be supposed to be in that part of her orbit designated by D, and moving 
in the direction D E, or from west to east. Let In he the orbit of the first satel- 
lite. Now, as the planet Jupiter would not be in opposition until the Earth would 
be between him and the Sun, as at E, the point D must be a situation of the Earth 
before opposition. In that case, as before stated, the immersions are seen ; for 



KOTES. 371 

the ray D I would cause the satellite to be projected on the surface of the heavens 
h e at h, and the planet Jupiter would appear to be among the stars at c. There- 
fore, the emersion would take place when the satellite was behind the planet, and 
therefore invisible to the observer at D. 

But let d represent the orbit of the third satellite. If the Earth were at D, the 
immersion at d would be visible, as well as the emersion at o ; so that the immer- 
sion and emersion would appear on the same side of the planet, after which the 
satellite would again be eclipsed by passing behind the body of the planet. For 
when the immersion happens, we see the satellite among the stars at a, and at the 
moment of emersion at h, after which it disappears a second time behind the 
planet's body. 

After opposition, or when the Earth is at F, the emersions of the satellites are 
visible, and the immersions invisible, being generally hidden by the body of the 
planet. When the Earth is at E, between the Sun and Jupiter, which is the place 
of opposition, the immersions and emersions are both invisible, because they occur 
immediately behind the body of the planet. The same is the case when the Earth 
is in superior conjunction, or in that part of its orbit opposite to E. 

When a satellite passes between the Earth and its primary, its small shadow 
may be seen traversing the planet's disc, causing to the inhabitants of those 
localities the phenomena of solar eclipses just as the shadow of the Moon eclipses 
the Sun when passing over the terrestrial surface. 

Before opposition, when the Earth is at D, the shadow of the satellite precedes 
the satellite itself. Thus, the shadow of the satellite at n falls on the planet and 
is visible from the point D before the body of the satellite arrives at a point be- 
tween D and Jupiter. The contrary is the case after opposition, or when the 
Earth is at F. The eclipse of one of Jupiter's satellites is visible at the same mo- 
ment at every point of the Earth's hemisphere which is turned towards it ; conse- 
quently, if these eclipses be accurately noted and compared with the times com- 
puted for their appearance for Washington, Greenwich, or any other meridian, 
they would serve to find the longitude of the observer ; for if the difference of time 
between the meridians of Washington or Greenwich and that of the observer should 
be found to be one hour, the difference of longitude must be 15°. This method of 
obtaining longitude is not available at sea, and is, besides, not the most accurate, 
though it may be relied on to within a quarter of a degree. 

The Kings of Saturn, their Nature and Constitution. 

Note 22, Page 90. 

When Galileo first saw Saturn through his telescope, he thought the planet was 
oblong ; but on further examination he was led to believe that it was composed 
of three globes — a large one in the centre, and a small one on each side of it. But 
it was not long before it was discovered that what he considered as two small 
globes were not really such, but that the globe of the planet was distinctly 
visible, having two projections, which he termed ansce, from their resemblance 
to handles. 

These observations were made known to Kepler about the year 1610; but it was 



372 bouvier's familiar astronomy. 

not tiU fifty years after, that Hevelius turned his attention towards the planet 
Saturn, and confirmed Galileo's observations. 

In 1659, Huyghens proclaimed to the world the discovery of Saturn's ring, in a 
work entitled Systema Saturnium. 

In the year 1665, the Messrs. Ball, of Minehead, England, saw the dark ellip- 
tical line which divides the ring into two parts ; and Whiston informs us that a 
fixed star was seen between the ring and the body of the planet. 

About the year 1675, Dominic Cassini, astronomer at the Paris Observatory, 
discovered that, instead of one, the planet was encircled by two luminous rings. 
In 1787, La Place, with his usual sagacity, says, in his "Theory of Saturn's 
Ring," that "it must be formed of several rings having nearly the same plane." 
This theory of La Place was very soon verified ; for in 1791, Sir William Herschel 
communicated to the Royal Society of London the result of his observations in June, 
1780, which was that he had discovered a second dark line upon the inner side of 
Saturn's ring, and on the preceding arm only, showing that it must be divided 
into three parts. This second line, however, soon vanished, which led astro- 
nomers to suppose the opening between the rings to be very narrow, and the rings 
eccentric. James Cassini accounted for the appearance and disappearance of the 
sides of the rings happening at irregular intervals to the annular surface not 
being in the same plane. 

Messier was of opinion, from certain inequalities of light, some parts appearing 
more luminous than the rest, that the surface of the rings must have fixed in- 
equalities, and those, too, of grand proportions, to be distinguished at such an 
immense distance, even with a good telescope. This irregularity of surface La 
Place adduces as necessary for the preservation of the equilibrium of the ring. 
He says : "I will add that these inequalities are necessary to maintain the equi- 
librium of the ring ; for if it were perfectly alike in all its parts, its equilibrium 
would be deranged by the slightest force, such as the attraction of a comet or a 
satellite, and thus the ring would be destroyed by being precipitated on the sur- 
face of the planet." La Place also considered it as necessary, in order to pre- 
serve the equilibrium of the ring, that its particles should not have a tendency to 
separate from it, — a condition only to be fulfilled by a rapid rotary motion round 
its centre of gravity and in its own plane. 

Mr. G. P. Bond, in a paper published in the American Astronomical Journal, of 
May, 1851, states, as his opinion, that the subdivisions of Saturn's ring are not 
permanent, from the fact that the division near the inner edge, observed by Sir 
"William Herschel on four difl'erent nights, in June, 1780, was never seen by him 
before that period or afterwards, although he devoted much time during thirty 
years to observations on the system of Saturn. 

In Gruithuisen's Astron. Jahrbuch, for 1840, mention is made of lines having 
been seen on both rings in 1813-14. Quetelet, at Paris, and subsequently Cap- 
tain Kater, have observed divisions in the outer ring of the planet. The latter- 
named gentleman saw three divisions in the outer ring in 1826, an account of 
which, together with full illustrations, may be found in vol, iv. part ii. of the 
Memoirs of the Astronomical Society. Professor Encke, at Berlin, in April and 
May, 1837, observed the outer ring nearly equally divided by a dark line. These 



NOTES. 6 i 6 

observations were published in the Astronomische Nachrichten, No. 338. De Vico, 
at Rome, Messrs. Lassell and Dawes, at Starfield, and others, also corroborate the 
foregoing testimony as to the subdivisions of Saturn's rings. 

It is true that neither Struve, Bessel, Sir John Herschel, and some others whose 
instruments are of a superior order, have ever seen more than one division in the 
ring. The subdivisions are not usually visible in both rings at the same time, which 
Mr. Bond conjectures is owing to some real alterations in the disposition of the 
material of the rings. Hence he infers they are not solid, but composed of matter 
in a fluid state, which, within certain limits, changes their form and position in 
obedience to the laws of equilibrium of rotating bodies. So that should any dis- 
turbances bring the rings together, the velocities at the point of contact being 
nearly equal, they would coalesce without disastrous consequences. 

Professor Peirce, in an ably-written paper published in the American Astrono- 
mical Journal, of June 16, 1851, sustains the above opinion of Mr. Bond. He 
says that Mr. Bond chiefly derived his argument for the fluidity of Saturn's 
ring from direct observation ; whereas he demonstrates that it cannot be solid 
from purely mathematical considerations. His words are — "I maintain uncondi- 
tionally, that there is no conceivable form of irregularity, and no combination of irre- 
gularities consistent with an actual ring, which would serve to retain it about the pri- 
mary if it were solid." 

One proof of the fluidity of the rings is, that although the diameter of the outer 
part of the ring has not been observed to change, the inner edge is contracting 
gradually. The measurement of the inner edge, according to Huyghens, in 1657, 
was 6''^-5. In 1695, Huyghens and Cassini found it to be 6^'' Bradley, in 1720, 
made5^^-4; and in 1799, Herschel's measurement was 5^''. According to the 
elder Strave, it was 4^^-36 in 1826 ; in 1838, 4^^-04, as measured by Encke and 
Galle ; and in 1851 it only measured 3''^-67, according to Otto Struve. 

Professor Peirce, judging from these data, concludes that the breadth of the 
ring is decreasing more rapidly than formerly, and that in about 80 years hence 
the ring will be ruptured. The planet acts no part, he thinks, in maintaining the 
equilibrium of the ring, but that the satellites are the agents which sustain its 
position. 

From the above it may be seen that the opinions of some of our greatest Ame- 
rican astronomers are favorable to the fluidity of Saturn's rings. Their density 
is supposed to be nearly that of water. 

In the introductory lecture to the course on Mechanical Science, delivered 
at the School of Mines, London, session 1851-52, Mr. Robert Hunt, Keeper of 
Mining Records, relates the following experiment,*illustrating the condition of 
bodies relieved from the influence of gravitation, which tends to prove that the 
remarkable phenomenon of luminous rings is due to the influence of motion ex- 
erted under peculiar conditions : If oil be dropped upon water, it swims ; if upon 
alcohol, it sinks ; but if we make a careful combination of water and alcohol, 
we obtain a fluid of the same specific gravity as the oil, and the globule of oil Avill 
swim in the very centre of the fluid, a perfect sphere. If into a properly-arranged 
glass box we pass a fine wire through the sphere of oil, and by means of a handle 

* This experiment was first mnde by M. J. Plateau, and published in Annales de Chiniie et de 
Vhysique, tome xxx. p. 203, and Mfimoires de I'Academie de Bruxelles. tomes xvi. et xxiii. 



^74 



BOUVIER S FAMILIAR ASTRONOMY. 



cause the globule to revolve slowly, the sphere becomes an oblate spheroid : by 
increasing the motion, we flatten it still more, until at a certain rate of revolution 
it becomes a disc, when a ring of oil is thrown off from the central globule, and 
although separated by intervening water, it revolves at precisely the same rate. 
It is highly interesting to find mechanical science lending its aid in explaining the 
grander phenomena of the creation. — Records of the School of Mines, vol. i. p. 71. 

There is some reason to suppose that the phenomenon of Saturn's ring was 
known in remote ages. In the "Indian Antiquities," by Maurice, is an engrav- 
ing of Sani, the Saturn of the Hindus, taken from an image in a very ancient 
pagoda, which represents the deity encompassed by a ring formed of two serpents. 

The Dark Ring of Saturn. 

Note 23, Page 90. 




NOTES. 6 t b 

In 1838, Dr. Galle, of the Berlin Observatory, noticed a gradual shading of the 
inner ring of Saturn towards the body of the planet. But it was not until 1850 
that the Messrs. Bond, of the Cambridge (Massachusetts) Observatory, succeeded 
in establishing the existence of a dark ring round the planet Saturn, a fact which 
hitherto had been only a matter of conjecture. It is not, as some have sup- 
posed, a subdivision of the luminous rings; for, unlike them, the light reflected 
from its surface is so feeble as to cause it to appear like a dark line across tlie 
planet's disc. The Rev. W. R. Dawes observed this dusky ring divided by a very 
narrow, darker line. 

Mr. Lassell made some observations of the rings of Saturn in the years 1852-53, 
during a sojourn in the island of Malta. He described the interior, or dark ring, 
as well defined against the sky, the shadow of the ball of the planet being dis- 
tinctly visible on it, although it was sufficiently transparent to permit the limbs 
of the planet to be seen through it. Mr. Lassell compares the appearance of that 
part of the dark ring which crosses the planet to that of a "crape veil." He 
therefore infers that its texture is of a semi-transparent nature, imperfectly trans- 
mitting the color or shade of the ground on which it is placed. The belts on the 
planet appeared of a dull, ruddy color, gradually shading into a greenish hue.* 

A remarkable feature of the dark ring was its transparency, the part crossing 
the ball being decidedly lighter in shade than the other parts ; the preceding and 
following limbs being distinctly visible, affording an indication of want of solidity, 
and corresponding to the requisitions of fluid matter. 

Satellites of Saturn. 

Note 24, Page 92. 

On the 25th of March, 1655, Huyghens discovered a small star at the distance 
of about 3'' from the planet Saturn, and on the west side of it. The telescope which 
he used on that occasion was 12 feet focal length, which he had constructed himself. 
The ring of Saturn was in the position to appear like a luminous line extending on 
opposite sides of the planet. Having noted the situation of this small body, he 
found on the following evening that from its position it must be a satellite. He 
computed its period to be nearly 16 days. This first-discovered satellite is the 
sixth in order from the planet, and is called Titan. 

When Huyghens made this discovery known, he announced this theory ; viz. 
that as the number of planets and satellites was now equal, and as the aggregate of 
both amounted to 12, which was universally admitted to be a perfect number, that 
the planetary system was complete, and thus ventured to predict that no more 
satellites would be discovered. 

In 1671, while the elder Cassini was examining the planet Saturn with a tele- 
scope of 17 feet focal length, he perceived a small star, which proved to be a satel- 
lite. This was Japetus, the eighth satellite in order from the planet, and the most 
distant from it. At its maximum brightness, Japetus appears like a star of the 
ninth magnitude. This satellite is only visible to us during that half of its revo- 



* Memoirs of Royal Ast. Soc. vol. xxi. part i. p. 151. 



376 bouvier's familiar astroxomy. 

lution lylien it is west of the planet ; and as the same phenomenon always occurs 
when it is in that part of its orbit, astronomers have concluded that, like oui* 
Moon, it revolves on its axis in the same time it revolves around its primary. 
The variations of brightness are supposed to be owing to some parts of its surface 
being less capable of reflecting light than others. 

The next satellite in order of discovery was Rhea, the fifth in order from the 
planet. Cassini, while searching for Japetus, which had become invisible, de- 
tected, with the aid of telescopes of 35 and 70 feet focal length, another satellite 
on the 23d of December, 1672. 

In March, 1684, Cassini, through his untiring perseverance and unwearied ob- 
servations, was rewarded by discovering Dione, the fourth satellite in order from 
the planet. 

In the same month, and nearly about the same time, he discovered Tethys, the 
third satellite in order from the planet. These two last-named satellites were 
detected by Cassini with object-glasses of 100 and 136 feet focal length, con- 
structed at Rome by Campini, Before the effects of chromatic aberration were 
obviated, it was found necessary to construct refracting telescopes of enormous 
length, which rendered them wholly unmanageable. Cassini and Huyghens 
simultaneously contrived means to obviate the difficulty by fixing the object-glass 
in a suitable position for observing a celestial body, and dispensing with the tube 
of the telescope altogether. 

The second satellite from the planet, called Enceladus, was first seen by Sir 
William Herschel in August, 1787, but was not certainly ascertained to be a satel- 
lite till August, 1789, after the completion of his great 40-feet reflector. As soon 
as this noble instrument was directed to the planet Saturn, six satellites were im- 
mediately visible, one of which proved to be Enceladus. This satellite can be de- 
tected only by means of the most powerful telescopes. 

In the following month, September, 1789, Sir William Herschel discovered the 
first satellite in order from the planet. This attendent moon is known by the 
name of Mimas. It can only be distinguished by means of the most powerful in- 
struments. Sir William Herschel described it as a "very small lucid point." On 
account of its vicinity to the planet, this satellite is often invisible throughout 
the greater part of each revolution. 

During a period of 59 years the planet Saturn was supposed to have only seven 
satellites. But on the 16th of September, 1848, Professor Bond, with the aid of 
the great Cambridge (Massachusetts) Refractor, discovered an eighth attendant 
on Saturn. This satellite, which is called Hj'perion, was seen three nights after 
by Mr. Lassell, of Liverpool; that is, on the 19th of the same month. This newly- 
discovered satellite is the seventh in order from the planet. 

The satellites of Saturn nearly all revolve in orbits that lie in the same general 
plane as the ring; therefore, when the ring is situated obliquely with regard to 
the Earth, so that its upper or under surface is towai^ds us, the satellites appear 
to be scattered confusedly around the primary instead of moving across in straight 
lines, as Jupiter's moons. But when the edge of the ring is turned towards the 
Earth, the moons seem to move in straight lines. The innermost of them has 
been seen by Sir William Herschel travelling along the edge of the ring, like a bead 



NOTES. 877 

of liglit moving on a string. Very powerful telescopes enable observers to witness 
the eclipses of Saturn's moons. 

In the year 1789, Sir William Herschel observed the shadow of Titan, the sixth 
satellite, transit Saturn's disc like a black spot. 

Distance of the Sun in Winter and Summer. 

Note 25, Page 107. 

We are farther from the Sun in June than in December ; first, because the 
Earth requires a longer time to move from the vernal to the autumnal equinox 
than from the autumnal to the vernal, causing our spring and summer to be nearly 
eight days longer than our autumn and winter. The reason why the Earth is 
longer in performing her journey from March, the time of the vernal equinox, to 
September, the time of the autumnal equinox, than from September to March, is, 
that from the vernal equinox the Earth is continually receding farther from the 
Sun, and therefore his attractive power is constantly diminishing, thereby de- 
creasing the velocity of her motion until she arrives at aphelion, or the farthest 
point from him. 

On account of the eccentricity of the Earth's elliptical orbit, any line (except 
the transverse axis) passing through the centre of the Sun must necessarily 
divide it into two unequal parts ; and by the laws of elliptical motion the Earth 
moves through these two unequal portions with unequal velocities. As the peri- 
helion always lies in the smaller portion, there the Earth's motion is most rapid. 

As it requires 20,984 years for the Earth's perihelion to make one complete 
revolution, (in which time it will successively occupy every point of the ecliptic,) 
it follows that in half that time, or in 10,492 years, the spring and summer in the 
southern hemisphere, which is now shorter, will then be longer than the autumn 
and winter, corresponding with the present length of our seasons in the north- 
ern hemisphere. 

Sir John Herschel has shown that the change in the perihelion can have 
no influence whatever on the temperature of the diflFerent portions of our globe ; 
for although the Earth is nearer to the Sun while moving through that part of her 
orbit in which the perihelion lies than in the other part, and consequently receives 
a greater share of heat, yet as it moves faster, it is exposed to the heat for a 
shorter time. In the other part of her orbit, on the contrary, the Earth receives 
fewer of the Sun's rays, but as its motion is slower, it is exposed to them for a 
longer time. 

The Sun's apparent diameter being greater in winter than in summer, is another 
proof of his distance from us being greater in summer. This foct is clearly proved 
by means of a micrometer. This is an instrument across which are extended very 
fine threads or wires, so adjusted as to be exactly parallel to each other, and mova- 
ble by means of a screw, so that they can be drawn close together or separated at 
pleasure, always, however, preserving their parallelism. AVhen this instrument is 
attached to the telescope, the disc of the Sun may be accurately measured by 
causing the wires to touch the one the upper and the other the lower limb of 
the disc. 



EOUVIER S FAMILIAR ASTRONOMY, 
Fig. 212. 





2 3 

Now if the -wu-es be adjusted so as to apparently touch each limb of the Sun on 
the 21st of December, as represented at 1, [fig. 212,) it Avill be found that by the 
21st of June they will be beyond the edge of the Sun's disc, as at 3, [fig. 212 ) 

Fig. 213. 




NOTES. 879 

If they are made to touch on the 21st of June, by December they will be found to 
be within the boundary of the Sun's disc, as at 2, {fig. 212;) thus showing con- 
clusively that the Sun's apparent diameter is greater in winter than in summer, 
which is owing to the difference of his distance from us in those seasons. 

In. fig. 213, if No. 1 be the apparent size of the Sun as seen from the planet 
Mercury, he would appear the size of No. 2 at Venus ; 3, at the distance of the 
Earth ; 4, at Mars ; 5, at the nearest of the asteroids ; 6, at the most distant aste- 
roid; 7, at Jupiter; 8, at Saturn; 9, at Uranus; and 10, at the planet Neptune. 

Day, as kept by Different Nations. 

Note 26, Page 110. 

Different nations begin their day at a different hour; thus, the Egyptians 
began their dly at midnight, and from them Hipparchus introdutjed that mode of 
reckoning into astronomy. But at present astronomers count their day from 
noon to noon, and not twice 12 hours, according to the vulgar computation. 

The method of beginning the civil day at midnight prevails in the United States, 
Great Britain, France, and indeed nearly throughout Europe. The Babylonians 
began their day at sunrise, from whence the hours calculated in this way are 
called Babylonic hours. In some parts of Germany and Italy they begin their 
day at sunset, and reckon twenty-four hours till it sets next day, calling that the 
24th hour: these are generally termed Italian hours. 

The Jews began their day at sunset, and divided it into twice 12 hours, as we 
do ; but they reckoned 12 hours for the period of sunshine, be it long or short, 
and 12 hours for the period of darkness ; so that their hours were continually 
varying in length, corresponding to the varying length of the days. The Romans 
also reckoned their hours after this manner. These hours were sometimes called 
planetary hours, because the five planets then known — namely, Jupiter, Saturn, 
Venus, Mars, and Mercury, together with the Sun and Moon — were regarded as 
presiding over the affairs of this world, and to take it by turns in the following 
order: First, Saturn, then Jupiter, Mars, the Sun, Venus, Mercury, and, lastly, 
the Moon. Each day of the week was named for that planet whose turn it was 
to preside the first hour. Thus, assigning the first hour of Saturday to Saturn, 
the second hour of that day would fall to Jupiter, the third to Mars, and so on, 
the twenty-second falling to Saturn again, the twenty-third to Jupiter, and the 
twenty-fourth to Mars ; so that the first hour of the next day (Sunda}') will fall 
to the Sun to preside. By the same reckoning, the first hour of the next day 
(Monday,) will fall to the Moon, the next to Mars, continued in the same rotation. 
Hence the days of the week came to be distinguished by the Latin names of Dies 
Saturnii, Solis, Lunae, Martis, Mercurii, Jovis, and Veneris; and among us by 
the names of Saturday, Sunday, Monday, &c. 

The term of a week or seven days was a period of time used by the Eastern na- 
tions generally. The Brahmins employed nearly the same denominations for the 
days of the week which we now use ; so did the Egj^tians, Jews, Arabians, and 
Assyrians, which may be considered as a proof of their common origin. 



380 bouvier's familiar astronomy. 



The Calendar. 

Note 27, Page 113. 

The notion of time was doubtless first impressed upon man by the regular re- 
currence of light and darkness, accompanied by activity and repose. This gives 
the idea of that portion of time known as a day. 

He also observed that comparative heat and cold, upon which depend the various 
productions of the Earth, recurred after a lapse of a certain number of these in- 
tervals of light and darkness, which gave him the notion of a year. 

In general, nations have marked this portion of time by some word indicating a 
return of the circle of the seasons. Thus, the Latin annus signified a ring ; and the 
Greek mouth? implies something returning into itself. The word year, as it exists 
in the Teutonic languages, of which our word year is an example, is said to have 
its origin in the word yra, which in the Swedish language means a ring, and is, 
perhaps, connected with the Latin gyrus, (a circle formed by wheeling round. ) 

The ancients marked the return of the seasons by the rainy and the dry season, 
or summer and winter ; also by the migration of the different kinds of birds. The 
rising and setting of certain fixed stars also served to indicate the different por- 
tions of the year ; thus the rising of the Pleiades in the evening showed the ap- 
proach of winter, and the heliacal rising of Sirius, which the Egyptians called 
Sothis, coincided with the rising of the Nile. Hesiod directs the husbandman 
when to plow and when to reap by the setting of the Pleiades. 

By such rude observations it was afterwards determined that the year consisted 
of at least 365 days. We learn from Herodotus that the Egyptians claimed the 
honor of discovering this number of days in the year, which they asserted they 
learned from the stars. The Jews also had a similar reckoning at a very early 
period. 

By this mode of computation the seasons would not return exactly to the same 
months of the civil year ; but instead of being fixed to certain months, after a 
lapse of time, winter would be found in autumn months and summer in the spring 
months. 

The Roman calendar, a term derived from the Latin calendce, was very rude in 
its structure until the time of Julius Caesar. The ancient Roman year consisted 
of 10 months, which, by the reformation of Numa Pompilius, was made to consist 
of 12. When Caesar became dictator, by the advice of Sosigenes, an eminent 
Alexandrian astronomer and mathematician, he adopted the mode of intercalation 
of one day in four years, which we still retain. This mode of computing time is 
called Old Style, and the calendar containing the account of time according to the 
Julian method of reckoning is called the Julian calendar, which came into use 
January 1, b. c. 45. The Julian epoch is that period included from the time the 
Julian calendar was first instituted to the time of Christ — viz. forty-six years be- 
fore Christ. 

The day of new moon immediately following the winter solstice in the 707th 
year of Rome was made the first of January of the first year of Julius Caesar. 
The 25th of December of his forty-fifth year is considered as the date of Christ's 



NOTES. 381 

nativity; and the forty-sixth year of the Julian calendar is counted the first 
of the Christian era. The year preceding the birth of our Saviour is called 
by chronologists the first year before Christ, but by astronomers it is called 
the year 0. 

In order to introduce the new system as established by Julius Caesar, it was 
necessary to enact that the previous year — namely, 46 B. c. — should consist of 445 
days; for which reason that year was called the "year of confusion," but by 
Macrobius more appropriately the " last year of confusion," 

The Gregorian year or calendar was established by Pope Gregory XIII., in 1582, 
for the purpose of correcting the errors of the Julian calendar. To this end. Pope 
Gregory, aided by the most celebrated mathematicians of his time, ordained that 
a day should be added to the month of February once in every four years ; and 
that on and after the 1600th year of the Christian era, and every fourth century 
thereafter, should be a bissextile or leap year ; thus every year divisible by 4 with- 
out a remainder should contain 366 days, q,nd all years divisible by 400 without 
a remainder should also contain 366 days. By this rule the error amounts 
to less than a day in 3600 years, which may be avoided by extending the rule ; 
that is, to make all years divisible by 4000 consist of 366 days, which would 
make an error of only a single day in 100,000 years. 

This mode of computation is called the Gregorian, or New Style, and is noW 
generally introduced into all the countries of Europe, except Russia. 

In order to make any date of the Old Style correspond with the New, we must 
now add to it twelve days. Thus, General Washington was born February 11, 
1731, Old Style, which, happening in the early part of the eighteenth century, 
eleven days only must be added, which makes his birthday fall on the 22d of 
February, 1732.* For in England, previous to the year 1732, the year com- 
menced on the 25th of March; therefore the 11th of February would be near the 
end of the year 1731, while according to New Style the year begins the first of 
January ; therefore the date of his birth would be near the beginning of the 
year 1732. 

The civil year is that which has been established by government for civil pur- 
poses, which consists of an even number of days, the odd hours and minutes not 
being reckoned. It consists of 365 days, except every fourth year, which con- 
tains 366. 

Albategnius, an Arabian prince, made observations at Aracte, in Chaldea, to- 
wards the beginning of the tenth century ; and by comparing his observations 
with those of former astronomers, he fixed the length of the tropical year to 365c?. 
5^. 46m. 245. In the year 1252, Alphonsine X., of Castile, obtained the assistance 
of the best astronomers of his age to arrange a series of astronomical tables, in 
which we find the length of the year to be 365c?. 5A. 49to. 16^., a very near 
approximation to the truth. These observations were called the Alphonsine 
Tables. 

The period of the duration of the tropical year has been determined by the fol- 
lowing observers, namely : 

* Olmsteacl's Astronomy, p. 47. 



382 eouvier's familiar astronomy. 

d. h. m. a. 

Nicolas Copernicus, in 1543 3i5 5 49 6- 

Tycho Brahe, in 1602 .'. 365 5 46 45 5 

Kepler, in the Tabulae Rudolphinae, in 1610 365 5 48 57-6 

James Cassini, in 1743 3G5 5 48 52-4 

Flamstead, First Astronomer Royal at Greenwich Observatory, 1680 365 5 48 57'5 

Halley, in his Astronomical Tables, about 1700 365 5 48 548 

Lacaille, in his Tables 365 5 48 49- 

Bessel, in 1830, for the year 1800 365 5 48 47-8 

The astronomical year begins on the 31st of December, at noon. 

Each year of our calendar, -which is almost exclusively in use throughout 
Christendom, consists of 12 unequal months ; but the Turks and Jews make their 
year to consist of 12 lunar months, or 354 days. A month, according to the Eng- 
lish law, is a lunar month, or 28 days, unless otherwise expressed, and a lease for 
12 months is only for 48 weeks.* But in some of the United States it has been 
enacted that whenever the term month has been used, it shall be construed to 
mean a calendar, and not a lunar, month. f 

Cycles. 

Note 23, Page 114. 

The cycle of the Sun, or 28 years, multiplied by the cycle of the Moon, or 
Metonic cycle of 19 years, make 532 years, which is called the Paschal cycle, be- 
cause it serves to ascertain when Easter occurs. This cycle was invented by Vic- 
torius of Aquitain, a. d. 463, and was in use until the Roman Empire became 
extinct. 

The ancients ascertained the fact that the new and full moons happen at very 
nearly the same times of the year after a period of nineteen years. This was 
marked by the Greeks with letters o^ gold. Hence the number of the year in this 
cycle is called the Golden Number. 

The solar and lunar years do not commence together till after nineteen years, 
during which time the solar years exceed the lunar. The number of days by 
which the solar year exceeds the lunar is called the Epact, which means some- 
thing added. 

The Roman Indiction was a period of 15 years, appointed by the Emperor Con- 
stantine, a. d. 312, for the payment of certain taxes from the subjects of the 
empire. 

At the Council of Nice, a. d. 325, it was decided that Easter-day is the first 
Sunday after the full moon which happens upon or next after the 21st of March. 
It follows, therefore, that Easter-day cannot take place earlier than the 22d of 
March, or later than the 25th of April, so that from one date to the other (both 
inclusive) are thirty-five days. The Number of Direction is that day of those 
thirty-five on which Easter Sunday falls. 

The Julian Period consists of the cycle of the Sun, or 28 years, multiplied by 
the lunar cycle, or 19 years, and that product multiplied by the Roman Indiction, 
or 15 years; which product is 7980 years. Thus 28 X 19 X 15 = 7980. This 

* 2 Blackstone's Commentaries, 141. f Bouvier's Law Dictionary, article Month. 



NOTES. 



383 



period is reckoned from 4713 years before Christ, when the three cycles are sup- 
posed to have commenced together, -which circumstance will not occur again until 
A. D. 3267. In order to find the Julian Period for any given year, add 4713 to the 
year of Christ. Thus 1860 -f 4713 = 6573, the Julian Period for the year 1860. 



Ecliptic. 

Note 29, Page 115. 
To an observer situated on the Sun, the Earth would appear to move in a th 
among the stars, which we designate by the name of the ecliptic. It is the con- 
tinuation of the plane of the Earth's orbit outwards from the Sun's centre until it 
reaches the sphere of the fixed stars. If a spectator on the Earth could see the 
stars in the same direction, but far beyond the Sun, that luminary would appear 
to be moving through the high constellations of Pisces, Aries, Taurus, Gemini, 
Cancer, and Leo during the spring and summer ; and through the low ones of 
Virgo, Libra, Scorpio, Sagittarius, Capricornus, and Aquarius in winter. Hence, 
to us in north latitudes, in summer the Sun seems to rise north of east on the 
horizon, crossing the meridian at a greater altitude than in winter, and to set 
north of west, remaining in sight longer than 12 hours; while in winter it rises 
south of east, crosses the meridian farther south than it does in summer, and 
sets south of west, remaining in sight less than 12 hours. 

Fig. 214. 



^^ 




Let fig. 214 represent the celestial sphere, P P being its poles ; then Q Q would 
be the equinoctial line, being exactly 90° from each pole. E would be the ecliptic, 
situated 66° 32' at its nearest approach to either pole, or 23° 28' at its greatest 
distance from the equinoctial. P Q E P Q E is a meridian which intersects the 
ecliptic at its greatest distance north or south of the equator. This meridian is 
called the solstitial colure ; and the points where it intersects the ecliptic are called 
the' solstices, because when the Sun is vertical at those points he is stai/ed from re- 



384 bouvier's familiar astronomy. 

ceding from the equatoi'. P 6 6 P is the equinoctial colure. This is a meridiai: 
which also passes through the poles, and through those points formed by the inter- 
section of the ecliptic and the equator. These points are called the equinoxes; 
for when the Sun is at those points, the days and nights are equal all over the 
Earth. Equinoctial is formed of the two Latin words cequus, equal, and noz, 
night, signifying equal night. 

Cause of the Tides. 

Note 30, Page 120. 
The alternate rise and fall of the tides twice in a lunar day is immediately 
owing to the force of gravitation exerted by the Sun and Moon. The action of the 
Sun and Moon, especially that of the Moon, disturbs the equilibrium of the ocean, 
the surface of which, were it not for these disturbing forces, would be an ellip- 
soid, flattened at the poles. 

If the Moon were to attract the centre of gravity of the Earth, and all the par- 
ticles of which it is composed, and also all the waters which cover the Earth, with 
an equal force, and in the same direction, the oceans would retain their equili' 
brium, and tides would be unknown. Owing to the Moon's attractive poAver be- 
ing inversely as the square of the distance, causing the diff'erence of the intensity 
and direction of the forces, and consequent disturbance of the equilibrium, the 
spheroidal form of the water is destroyed. 

The particles of water under the Moon are more attracted than the centre of 
gravity of the Earth ; consequently, they have a tendency to leave the surface of 
the Earth, and are thus heaped up on that side under the Moon, but are retained 
by their gravitation, which is lessened by the lunar attraction. But the centre 
of the Earth is more powerfully attracted than the particles of water in the hemi- 
sphere opposite to the Moon, so that the Earth has a tendency to leave the waters, 
but is retained by gravitation, which, however, is diminished by the Moon's 
attraction. Thus it will be seen why there are two tides at the same time on op- 
posite sides of the Earth ; the waters under the Moon are heaped up, forming a 
tidal wave ; and those in the opposite hemisphere are, at the same time, raised 
above the general surface ; for the diminution of the gravitation of the particles 
in each position is almost the same. 

Suppose a particle to be at a, immediately under 
the Moon M ; it will be as much more attracted than 
the centre of gravity of the Earth as the square of 
c M (the distance of the Moon from the centre of the 
Earth) is greater than the square a M, the distance 
of the Moon from the particle at a. Hence the par- 
ticle has a tendency to leave the Earth, but is re- 
tained by its gravitation. 

In twelve hours the particle a is brought to h by 
the rotation of the Earth on its axis, and is then in 
the opposite hemisphere from the Moon. The Moon 
attracts it now less powerfully than it attracts the centre of the Earth, in the 




NOTES. 385 

ratio of the square of the distance from c M (the distance of the centre of the 
Earth from the Moon) to the square b M, the distance of the particle b from the 
Moon. The Earth has now a tendency to leave the particle b, but its gravitation 
retains it. Thus, when the particle is at a, the Moon draws it from the Earth ; 
and when it is at b, it draws the Earth from the particle ; in both instances pro- 
ducing an elevation of the particle above the surface of equilibrium. 

The action of the Moon on a particle at d or c, 90° distant from a, may be re- 
solved into two forces — the one in the direction of the radius of the Earth dc ov ec, 
and the other in the direction of a tangent to the surface, wnich is at right angles to 
the Earth's radius. The latter force attracts the particle towards the Moon M, and 
causes it to move along the surface of the Earth towards a, so that there is a de- 
pression of the water at d and e at the same time that it is high water at a and b. 

Solar and Lunar Tidal Waves. 

Note 31, Page 120. 

The Sun attracts as powerfully as 354,936 earths would, if they held his place ; 
the Moon attracts but as an 80th part of the Earth ; the Sun, however, is 400 
times farther from the Earth than he is from the Moon. Now, the amount of the 
solar and lunar tidal waves is due to the difference with which two opposite sides 
of the Earth are influenced by solar and lunar attraction ; this difference is much 
less in the case of the large, remote Sun, than in that of the Moon, which is com- 
paratively small and near, because 8000 miles (the Earth's diameter) is a much 
more trifling quantity, compared with 95,000,000 of miles, than when compared 
with 240,000 miles. Thus, the great mass of the Sun makes but a small tidal 
wave, while the small mass of the moon makes a larger one. 

The solar tidal wave is so much less than the lunar tidal wave that it is disre- 
garded, excepting as a quantity that sometimes augments and sometimes diminishes 
the more important lunar rise. If the Earth were at perfect rest, and surrounded 
everywhere by a thin, even coating of water, the Moon's attraction would so alter 
the form of the fluid investment, that it would assume the outline of an ellipsoid, 
116 inches wider one way than the other. The Sun would, under the same circum- 
stances, make the water find a permanent position of rest in an ellipsoidal outline, 
45 inches wider one way than the other. By adding together the half of each of 
these quantities, 81 inches is obtained for the absolute height to which the com- 
bined influence of the Sun and Moon can raise the surface of the ocean on one 
side of the Earth. 

• 

Spring and Neap Tides. 

Note 32, Page 123. 
The height of the tides is much increased in syzigies, or at the time of new and 
full moon ; for when the Moon is new, the Sun and Moon are in conjunction, and 
when she is full, they are in opposition ; and therefore are, in both cases, in the 
same meridian. In each of these positions— that is, new or full moon — the at- 
tractions of the Sun and Moon are combined, which produces the highest or spring 
tides under that meridian, and the lowest in those points 90° distant. Th« higher 

25 



386 



BOUVIER S FAMILIAR ASTRONOMY. 



the sea rises in flood tide, the lower it falls in the ebb. The neap tides happen 
when the Moon is in quadrature, or 90° from the Sun ; and as the solar and lunar 
attraction is not exerted in the same direction, the neap tides do not rise so high 
nor sink so low as the spring tides. 

But the spring tides, which happen when the Moon is in perigee, are con- 
siderably higher than those which occur when she is in any other part of her 
orbit, especially that point most distant from the Earth. As the Moon is once 
new and once full in every month, there must be two spring tides in every luna- 
tion. The greatest tide occurs when a new or full moon happens when the Moon 
is at the point of her orbit nearest to the Earth, and the Sun vertical at the equa- 
tor ; hence, at the time of the equinoxes, when the Moon is in perigee, the highest 
tides occur. 

The above description of the phenomena connected with the progress of tidal 
waves only applies to the open ocean. In narrow channels the progress of the 
wave is impeded by mechanical obstacles ; hence, every particular port in confined 
seas has its own especial law for the time of high water. The high tide at 
Brighton, on the coast of England, for instance, occurs half an hour later than it 
does at Dover ; and at Falmouth, six hours later. The wave which enters the 
English Channel from the German Ocean requires eight hours to travel from Dover 
to Bristol. The channel of the river Amazon is so long, that there are seven high 
and seven low tides within its banks at the same time. 



Inclination of the Moon's Orbit. 

Note 33, Page 126. 
Place a plate (which will represent the Moon's orbit) half over the edge of a 
round table ; tilt the plate a little, so that half may be above and half below the 
top of the table. The surface of the table will represent the Earth's orbit, and 
the inclination of the plate to the surface of the table will then represent the in- 
clination of the Moon's orbit to the Earth's orbit. 

Fig. 216. P 




In the figure let E 1 represent the plane in which the Earth revolves about the 
Sun, and c c the concave surface of the heavens ; M m will represent the plane in 



NOTES. 387 

which the Moon revolves about the Earth. The line M m is inclined to the line 
E 1 by the angle M E 1. When the Moon is at M, or that part of its orbit that is 
nearest to the Sun, an observer on the Earth sees it at 2 in the heavens, along the 
line E 2 ; but he sees the Sun at 1 in the heavens, along the line E 1 ; consequently, 
when the Moon's orbit is placed as represented in the figure, the Sun cannot dis- 
appear behind the Moon. 

Motion of the Moon's Nodes. 

Note 34, Page 127. 

Although the Moon's orbit is elliptical, it is not a, perfect ellipse, its deviation 
from that figure being observable in every lunation. 

When the Moon performs a sidereal revolution, it does not return exactly to the 
same star again from which it set out ; thereby indicating a continual change in 
the plane of its orbit. The points where the Moon's orbit crosses the ecliptic, called 
the nodes, are continually changing their places by a constant retrograde motion, 
which amounts to about 3'' 10'' ''•64 on an average daily. In the period of 18 years 
and 219 days the nodes perform one complete revolution round the ecliptic, as 
may be seen by reference to the following figure : Y\^. 217. 

Let represent the position of the Earth, and 

the dotted line bad the plane of the ecliptic in- ^ JTT^ ...^^^ 

tersected by the Moon in its revolutions round X--'" ^ " ~^^ 

the Earth. BCDEF represents the Moon's /' nj^ ------^t-------^'"""^''''''^ V\ 

path, which, crossing the Earth's orbit at A, may v7 \~Z-^^ /• 

be called the Moon's ascending node. It will be \ ^^K^y^^ a y''' 

C E^^ .^. -^y^ 

seen that the Moon's orbit is not in the plane of ^ — ^^ 

the Earth's orbit, for the Moon moves along the 

curve A B, and intersects the Earth's orbit at C instead of a, which would be in 
the opposite point, or 180° distant from the node A. So that the descending node 
C is less than 180° from the ascending node A. The Moon on leaving C pursues 
the path C D E F; and instead of intersecting the ecliptic at c, which is 180° from 
C, it cuts it at E, which is behind the point c in longitude. In this manner the 
nodes of the Moon's orbit retrograde, until, after the lapse of nearly 19 years, they 
return to the same place again. 

Transits of Mercury. 

Note 35, Page 136. 

The earliest recorded observations of the planet Mercury date back as far as 
B. c. 265, on the 19th of the Egyptian month Thoth. 

The Chinese have recorded observations of this planet as remote as the second 
century of our era, the accuracy of which have been verified by modern astro- 
nomers, among whom is M. Le Verrier. 

The first authentic account which we have of the phenomenon of a transit of 
Mercury was that of November 7, 1631, as computed by Kepler, and observed by 
Gassendi, at Paris. 

In November, 1651, Shakerly, or Schakerlaus, made a voyage to Surat, in the 



388 bouvier's familiar astronomy. 

East Indies, in order to witness a transit of Merciu-y which would not be visible 
lu England. 

Hevelius and Halley also observed transits of Mercury. The latter conceived 
the idea of finding the Sun's parallax by accurate observations of the ingress and 
egress of the planet, taken at places widely asunder ; but it has been found that 
the difference of the parallaxes is not sufficiently large to answer the purpose. 
The transits of Venus are now carefully noted by astronomers for determining the 
solar parallax, which can be found very accurately by these means. 

Lalande, when at the advanced age of 70 years, observed the transit of Mercury 
which occurred on the 8th of November, 1802. To use his own words — " The 
passage of Mercury over the Sun's disc was observed this morning for the nine- 
teenth time. The weather was exceedingly favorable, and astronomers enjoyed, 
in the completest manner, the sight of this curious phenomenon. I was the more 
anxious to have a view of it, as I shall never see it more." 

During the political confusion attending the French Revolution, Lalande busied 
himself in computing and arranging tables, the correctness of which was proved 
at the time of this transit. On being congratulated for having escaped the fury 
of those times, he wittily replied — " I may thank my stars for it." 

Various phenomena have been recorded by astronomers during their observations 
of the transits of Mercury. Different observers at the same station have given 
different results of their observations with several kinds of telescopes. Some have 
seen the planet "pear-shaped" as it advances on the Sun, just previous to the in- 
ternal contact. By others a luminous bead of light has been observed on the disc 
of the planet when projected on the Sun. Luminous rings have also been fre- 
quently seen surrounding the planet ; at other times, dark or hazy rings have been 
observed. Many of these appearances are no doubt due to optical illusions. 

American Observations of a Transit of Venus. 

Note 36,. Page 138. 

The American Philosophical Society of Philadelphia appointed a committee of 
its members to take observations of the transit of Venus, which was to take place 
in 1769. 

This committee consisted of William Smith, D.D., Provost of the college of 
Philadelphia ; John Lukens, Esq., Surveyor-General of Pennsylvania ; David Rit- 
tenhouse, A.M., of Norriton, (Norristown;) and John Sellers, Esq., Representa- 
tive in Assembly for Chester county. 

This was one of several committees appointed by that Society for this purpose, 
all of whom labored under great discouragements for want of proper apparatus, 
especially good telescopes with micrometers. A part of this discouragement was 
removed, for the Provincial Assembly generously voted the purchase of one of the 
best reflecting telescopes, with a Dollond's micrometer ; and likewise the sum of 
one hundred pounds, for the erection of an observatory and other incidental ex- 
penses. These instruments were for the use of the Philadelphia Observatory. 
But the Norristown Observatory was not yet properly furnished with instruments. 

Previous to the appearance of the transit, Thomas Penn, Esq., one of the pro- 



NOTES. 389 

prietaries of the province, wrote a letter to the Philosophical Society, at the re- 
quest of the Rev. Nevil Maskelyne, then Astronomer Royal at the Greenwich 
Observatory, desiring "that the transit might be accurately observed in Pennsyl- 
vania, as the situation was especially favorable." He also sent printed directions 
for noting the observations. 

Incited by this letter of the great English astronomer, they sent over an order 
for a two and a half feet reflector and a Dollond's micrometer, to be sent from 
London as promptly as possible. The instruments were immediately forwarded, 
with a letter from Thomas Penn, which concludes as follows : 

"I have sent, by Captain Sparks, a reflecting telescope, with Dollond's micro- 
meter, exact to your request, which I hope will come safe to hand. After making 
your observation with it, I desire you will present it, in my name, to the college. 
Messrs. Mason and Dickson (Dixon) tell me they never used a better than that 
which I formerly sent to the Library Company of Philadelphia, with which a good 
observation may be made, though it has no micrometer." 

The transit of Venus was also observed by the Rev. Samuel Williams, A.M., at 
Newbury, (Newburyport,) in Massachusetts. 

The following is a portion of a letter received by Thomas Penn, Esq., from the 
Rev. Nevil Maskelyne, showing the American observations of the transit were 
highly thought of in Europe : 

Greenwich, Aug. 2, 1769. 

Sir : — I thank you for the account of the Pennsylvania observations, (of the 

transit,) which seem excellent and complete, and do honor to the gentlemen who 

made them, and those who promoted the undertaking. * -x- ^ * ■h- * * 

I am, sir, your very humble servant, 

Nevil Maskklyne. 
To the Hon. Thomas Penn, Esq. 

It is said that the care and responsibility which Mr. Rittenhouse experienced in 
preparing for this transit, and the intense feeling produced on the happy realiza- 
tion of his anticipations, had such an effect upon him that, at the conclusion of 
his observations, he fainted. 

Orbits of Comets. 

Note 37, Page 141. 

In the early ages of the world, comets were regarded as omens of war and pesti- 
lence, subject to no laws, but permitted by the Creator to rush wildly through 
space. By some they were thought to be meteors, which were kindled into a 
blaze as soon as they came within the atmosphere of the Earth, by which means 
they were consumed. 

They were known to move in elliptical or long regulated orbits, even as early as 
the time of Appolonius Myndius, about b. c. 300. But in their motions they pre- 
sented strikingly diflTerent characteristics from the planets. 

Tycho Brahd supposed them to revolve in circular orbits beyond that of the 
Moon, while Kepler affirmed that they moved in straight lines. Hevelius, towards 
the middle of the seventeenth century, was the first who remarked that the orbits 
of comets were curved towards their perihelion, the concave side beina; turned 



390 bouvier's familiar astronomy. 

towards tlie Sun, He, as well as Borelli, an Italian astronomer who flourished 
about the same time, conceived that the orbits of comets might be either parabolic 
or elliptic. But their orbits were unknown until Dorfel, a Saxon astronomer, in 
1681, proved that the orbits of many comets are parabolas, with the Sun in 
the focus. Newton was led, by his discovery of gravitation, to conclude that 
comets move in conic sections ; and that, on account of their great eccentricity, 
they might be assumed to be parabolas near the perihelia. 

Since the discovery of the law of gravitation, it has been determined that any 
body revolving round the Sun must move in one of those curves called the conic 
sections — namely, the circle, the ellipse, the parabola, or the hyperbola — and that 
the Sun must be situated in one of the foci of the curve. The orbit may vary in 
magnitude, position, or direction, but it must be one of the above-mentioned curves. 

The orbits of the comets do not coincide with the plane of the ecliptic, but may 
be found at every possible angle with it. Unlike the planets, the comets do not 
move in the same direction round the Sun — that is, from west to east — but revolve 
in all directions and with all possible inclinations. 

Thus it may be seen that the orbits of the comets, although subject to the law 
of gravitation, are nevertheless very different from those of the planetary bodies 
of our system. 

Comets which visit the Sun but once, and then sweep off into space, (perhaps to 
visit other systems.) have hyperbolic orbits ; for the extremities of hyberbolas run 
off from each other forever. Hyperbolically-moving comets must be looked 
upon as strangers to our Sun's system ; for possibly all comets may be wanderers, 
passing from sun to sun, and only sojourning near to one system for a time, when 
caught by the influence of its attraction. A comet moving in a hyperbolic path 
may approach so near to some planetary body as to have its velocity of motion 
retarded, and its hyperbolic path converted into an elliptical one. If the velocity 
of a body moving in a hyperbola be diminished, the motion at once becomes ellip- 
tical ; and if, on the other hand, the velocity of a body moving in an ellipse be 
increased, the motion becomes hyperbolic. It is known that the comet of 1770, 
commonly called Lexell's comet, has been drawn into a new path in consequence 
of having passed very near to the planet Jupiter. 

Discovery of Telescopic Comets. 

Note 38, Page 143. 

Nearly every year furnishes us with the discovery of one or more new comets. 
Astronomers are now on the alert to detect these numerous wanderers as soon as 
they appear within the limits of telescopic vision. 

In 1840, the late king of Denmark offered a prize gold medal to the discoverer 
of every new comet. On the first of October, 1847, Miss Maria Mitchell, of Nan- 
tucket, perceived a nebulous body a few degrees from the North Pole, which, on 
the following evening, had so much changed its place as to confirm the suspicion 
of its being a comet. This discovery having been made known to the king of 
Denmark, the gold medal was awarded to Miss Mitchell, who was the first Ame- 
rican, and the only lady, who has ever received it. This comet was also seen on 



/ 



NOTES. 391 

the 3d of October, by De Vico, at Rome ; on the 7th, by the Rev. W. R. Dawes ; 
and on the 11th, -by Madame Riimker, at Hamburg. 

Every comet is carefully noted as soon as observed, and its elements recorded ; 
BO that in future time these observations may lead to important discoveries in 
coraetary astronomy. 

Tails of Comets. 

Note 39, Page 147. 
The comet's tail cannot be an emanation drawn out by the Sun's attractive 
influence, for it is always turned away from the Sun. Neither can it be matter 
left behind during the comet's rapid motion, for after the perihelion passage it goes 
before the nucleus instead of following it. It seems rather to be some mysterious 
exhalation, raised by the power of the Sun's influence, and then subjected to the 
rule of some strange and yet unknown agency, which must, however, be of a 
difi"erent nature from gravitation. The tail of the comet of 1680 shot out through 
its hundred millions of miles in length in two days. The tail of the comet of 1843 
could not have had inferior dimensions, and yet this wonderful appendage was 
brandished round through half a circle while the comet passed through its peri- 
helion curve, the end of the tail sweeping through a bending line some 300,000,000 
of miles long in two hours of time. Herschel beautifully likens this comet's tail 
to a negative shadow, as it moved through its perihelion passage. 

Halley's Comet. 

Note 40, Page 152. 

On the appearance of Halley's comet, in 1835, it seemed to be a spherical mass 
of vapor without a tail ; but as it approached its perihelion, its tail increased to 
thirty or forty degrees in length. 

Soon after its appearance in that year, it was observed by M. Struve, at Dor- 
pat; the nucleus resembled a fan-shaped flame, emanating from a bright centre, 
after which it assumed the appearance of a red-hot coal of an oblong form. In 
a few days its appearance is described as resembling the stream of fire which 
issues from the cannon's mouth after a discharge, when the sparks are driven 
backward by a violent wind.* 

M. Struve saw distinctly a central transit of a star of the ninth magnitude 
through the nucleus of the comet. The star remained constantly visible, without 
any considerable diminution of light ; and instead of being eclipsed, the nucleus 
of the comet became invisible, owing to the superior brightness of the star. 

But great and sudden changes have been observed in this comet, sometimes in 
very short intervals of time. On one occasion, the nucleus appeared clear and 
well defined; and in the space of a few hours it became obscure and much 
enlarged. On another occasion, luminous brushes or sectors appeared, diverging 
from its centre through the nebulosity. M. Struve describes the nucleus of the 



* For further information on this subject, see a Treatise on Comets, by J. Russell Hind, and Hei^ 
Bchel's Observations at the C;ipe of Good Hope. 



392 bouvier's familiar astronomy. 

comet, as obseryed bj him on one occasion, as elliptical, and like a burning coal, 
out of which there issued, in a direction nearly opposite to the tail, a divergent 
flame, varying in intensity, form, and direction, and sometimes appearing double, 
and resembling a luminous gas issuing from the nucleus. M. Arago saw three 
divergent flames opposite to the tail. Sir John Herschel, at the Cape of Good 
Hope, saw these luminous fans. When it appeared in the year 1682, brilliant 
flames similar to those above described, were observed by Hevelius. 

On the appearance of this comet in 1682, Halley, at the suggestion of Newton, 
searched all ancient and modern records in order to discover any similarity 
between the elements of former comets and the one then visible ; and from an 
examination of all the facts, he ventured to predict its reappearance about the 
year 1758 or 1759. 

As the time drew near for the verification of this great prediction, astronomers 
were all anxiety, for that great philosopher (Halley) did not live to see the fulfil- 
ment of his prophecy. In order to compute the disturbing effects of the primary 
planets upon this comet, it was necessary to calculate them for a period of 150 
years, which is equal to two revolutions of the comet. This task was accom- 
plished by Clairault and Lalande. The latter undertook the arithmetical calcula- 
tions, assisted by Madame Lepaute, the wife of a Parisian watchmaker. " During 
six months," says Lalande, "we calculated from morning to night, sometimes 
even at meals ; the consequence of which was, that I contracted an illness which 
has changed my constitution for the remainder of my life. The assistance ren- 
dered by Madame Lepaute was such that without her we never should have dared 
to undertake this enormous labor, in which it was necessary to calculate the dis- 
tance of each of the two planets, Jupiter and Saturn, from the comet, separately 
for every successive degree, for 150 years." 

The name of Madame Lepaute is not mentioned by Clairault in his memoir 
announcing the completion of their labors, which Lalande said was owing to his 
fear of arousing the jealousy of another lady to whom he was attached. 

Madame Lepaute assisted Lalande in many other calculations, and by this 
means so weakened her sight, that finally she was forced to relinquish all arith- 
metical labors. 

These stupendous calculations being finished, Clairault announced the result of 
their joint labors in a communication to the Academy of Sciences of Paris, in 
which he predicted the next perihelion passage of the comet on the 18th of April, 
1759, but which he afterwards changed to the 11th of that month. It actually 
passed its perihelion on the 13th of March, within 22 days of the time assigned 
by Clairault. 

Its next appearance, in 1835, happened within a few days of the predicted time. 

Lexell's Comet. 

Note 41. Page 154. 
Lexell's comet of 1770 passed within six times the Moon's distance from the 
Earth, and was considerably retarded in its motion in consequence of the terres- 
trial attraction for its mass. But it did not in return produce the slightest effect, 



NOTES. 393 

even in the tidal rise of our ocean. Had the comet's mass been equal to that of 
the Earth, the length of the year would have been increased three hours by its 
retarding power over the Earth's motion. The length of the year was not, how- 
ever, increased so much as the smallest fraction of a second ; and hence it can be 
shown that the comet's mass could not be so great as a five-thousandth part of the 
Earth's mass. The extreme thinness of the comet's substance seems to be due to 
the absence of any central dense matter capable of controlling the elasticity of the 
(.lometic material. If the Earth were to retain its present size, and yet be reduced 
to the one-thousandth part of its actual mass, the atmosphere would leap out to 
considerably more than one thousand times its present dimensions. Newton has 
shown that a globe of air of an inch diameter, if reduced to the density it would 
have when removed four thousand miles from the Earth's surface, would be suffi- 
cient to fill a sphere exceeding in its circumference the orbit in which Saturn 
moves ; and of some such material as this the cometic vapor must be composed. 



Precession of the Equinoxes. 

Note 42, Page 165. 

The protuberant matter of the Earth's equator exercises a disturbing influence 
over the Moon's motions ; and the Moon in return attracts it, and deranges the 
Earth's rotation, to a certain extent, in consequence. It is the Moon's attracting 
the protuberant zone of the Earth's equator, sometimes one way and sometimes 
another, that gives the vacillation to the Earth's rotation, causing the precession 
of the equinoxes. The Sun also exercises a disturbing influence over the pro- 
tuberant mass of the terrestrial equator. 

This vacillation, or conical sweep of the Earth's axis, is caused by the masses of 
the Sun and Moon attracting the protuberant matter at the equator of the Earth, 
sometimes one way and sometimes the other way, accordingly as they are placed 
with regard to it. The immediate consequence of this conical sweep of the Earth's 
axis is to cause the equator to intersect the ecliptic in some new point at each 
revolution. The points of intersection travel backwards upon the ecliptic, or 
from east to west, fifty seconds and a tenth every year. After 25,868 years, thesi 
points of intersection will have gone completely round the circle of the ecliptic. 
As the two points where the plane of the equator intersects the plane of the 
Earth's orbit [i. e. the ecliptic) are called the equinoctial points, this travelling 
back of the points of intersection is called the ''precession of the equinoxes.'' 

The precession of the equinoxes was discovered by Hipparchus, the most cele- 
brated astronomer of antiquity. On comparing his observations of the longitudes 
of several stars which were made about 128 years before Christ, with those made 
by Timocharis and Aristillus 155 years earlier, he found that in that interval the 
longitudes of the stars liad increased more than could be accounted for by any 
errors of those astronomei's. 

By comparing the eclipses of the Moon of his own time with those observed by 
Timocharis, he found that the longitudes of all the stars increase perpetually ; 
wliich is owing to the precession of the equinoxes. This motion lias carried the 



394 bouvier's familiar astronomy. 

stars through one sign of the zodiac from his time to ours ; and, in order to com- 
plete one revolution, would require no less than 25,868 years. 

As the equinoxes have retrograded about 30°, or one sign, since the time of 
Hipparchus, it follows that the stars which were then in Aries are now in Taurus, 
and those which were in Taurus have receded into Gemini, and so on. Or, more 
properly, the sign Aries is now in the constellation Pisces, and the sign Taurus in 
the constellation Aries, &c. 

It should be distinctly understood that the attributed changes in the positions 
of the heavenly bodies, owing to the retrogradation of the equinoctial points, are 
entirely arbitrary ; for if a fixed star had been adopted as the point from whence 
to reckon longitudes and right ascensions, there would have been no such thing 
as precession ; all the longitudes and right ascensions would have remained con- 
stant, or nearly so, for ages. 

Nutation. 

Note 43, Page 166. 
All the heavenly bodies, whether fixed or in motion, are affected to a certain 
extent by the nutation of the Earth's axis; but the amount of variation can be 
easily calculated for any required time. 

Science is indebted to Dr. Bradley, formerly Astronomer Koyal at Greenwich, 
for his discovery and explantion of this apparent displacement in the situations 
of the celestial bodies. 

The Moon being so near the Earth occasions disturbances in the motions of 
both bodies. Thus, the action of the Moon on the matter of the Earth's equator 
produces a nutation in the axis of rotation ; and the reaction of that matter on 
the Moon is the cause of corresponding nutation in the lunar orbit. 

In fig. 218, let U ^p Q and T fp E be the planes 
of the equinoctial and ecliptic, and P their 
poles. Then suppose 0, the pole of the equator, 
to revolve with a tremulous or wavy motion in 
the little ellipse Oc db'm about 19 years, both 
motions being very small, while the point a is 
carried round in the circle A a B in 25,868 years. 
The tremulous motion may represent the half- 
yearly variation, the motion in the small ellipse 
gives an idea of the nutation, and the motion in 
the circle a A B arises from the precession of the 
equinoxes. The greater axis c? of the small 
ellipse is 18^''-5, its minor axis 6 c is 13^''*74. 
Thus the line of the poles describes an acute conical surface once in every 19 
years, of which the vertex is at the centre of the Earth ; and the intersections with 
the celestial sphere are two equal ellipses, whose transverse and conjugate axes are 
respectively 18^^-5 and 13^^-74: the former being always directed towards the 
poles of the ecliptic. 




NOTES. 395 

Refraction. 

Note 44, Page 168. 

The laws of refraction, although imperfectly understood until the time of Des- 
cartes, in the first half of the seventeenth century, were the object of inquiry 
among the ancients. Archimedes, who lived 291 b. c, is said to have written a 
treatise, entitled '*0n a Ring seen under Water," in which he treats of some of 
the phenomena attendant on refraction. Seneca remarks that an oar in clear 
water appears broken; but not being able to give a sound philosophical solution 
to the problem, wisely concludes that "nothing is so fallacious as our sight." 

Ptolemy, who lived in the second century, as well as many other ancient 
observers, remarked the effect of refraction on the rising and setting of the heavenly 
bodies. As he was one of the most distinguished scientific men among the ancients, 
the discovery of refraction was by some imputed to him. He certainly made some 
valuable observations in order to determine its law. Many of the instruments 
which he used were of his own invention. He composed the Jwta^iz fxyyu^, Syn- 
taxis 3Iegale, the " Great Composition," which is the only complete sj^stematic 
work on astronomy ever produced by the ancients. To the word Mcgale the Ara- 
bians added the particle "Al," and called it ^^ Almagliesli,'''' which we have cor- 
rupted to Almagest. This work was a collection of a great number of the obser- 
vations made by Hipparchus, as well as other ancient astronomers; and also 
contains problems relating to astronomy and geometry. It contains a catalogue 
of the fixed stars then known, with their places, as well as numerous records 
and observations of eclipses, the motions of the planets, &c., all of which are very 
valuable to the astronomer. A very excellent edition of this work has recently 
appeared in Germany. 

The first sound views upon the subject of refraction occur in the works of 
Alhazen, an Arabian mathematician, who flourished about a. d. 1000, In lib. vii. 
he asserts that ^'■refraction takes place towards the perpendicular,''^ which he proves 
by experiments, and which is certainly one step towards the discovei-y of the laws 
which govern it. 

Roger Bacon, a Franciscan monk, gives a tolerably correct explanation of the 
effect of a convex glass ; and Vitellio, a Pole, who lived in the thirteenth century, 
wrote an excellent treatise on optics, and explained many of the phenomena pro- 
duced by refraction. 

Tycho Brah^ made some corrections for atmospherical refi-action ; but from all 
the facts which had yet been collected no exact laws had been traced. In 1604, 
Kepler published a " Supplement to Vitellio," in which he nearly stumbled on the 
law of refraction ; but after devising an almost endless variety of formulce, (as 
was his custom,) he failed to discover it He tried numerous constructions by 
triangles, conic sections, &c. without success, and at last was obliged to content 
himself with an approximate rule. 

The law of sines was discovered by Willebrord Snell, a Butch mathematician, 
about 1621, and was afterwards published by Descartes as his own. This last- 
named philosopher, however, gave the first correct explanation of the rainbow. 
At length, in 1672, the immortal Newton perfected the theories of his prede- 



396 bouvier's familiar astronOxMY. 

cessors, and gave the true law of refraction, -which is, that in the same medium 
'■Hhe ratio of the sines of the angles of incidence and refraction is constant;^' but thi? 
ratio varies with the refracting medium. 

Many curious and interesting phenomena are produced by the refraction of 
light. An instance is related by William Latham, Esq.,* in which he saw the 
French coast from the town of Hastings, which is situated on the southern coast 
of England. The French coast is more than 50 miles from Hastings, yet the 
different towns on the opposite shore were distinctly visible ; and by the aid of a 
spy-glass, the French fishing-boats were plainly distinguished. This curious phe- 
nomenon continued for more than three hours. As the convexity of the Earth 
would conceal the French coast, it proves that the peculiar refractive power of the 
atmosphere on that occasion brought it into view by bending the rays of light, 
and thereby raising it above the horizon to the observer on the English coast. 
Dr. Wollaston proved such phenomena to be owing to the refraction of the rays 
of light through the media of different densities. One of his experiments is that 
of heating a poker red hot and observing any object in a line with it ; two images 
are seen — the one direct, the other inverted. 

He produced the same effect by a saline or saccharine solution with water, and 
spirit of wine floating upon it. In 1818, Captain Scoresby, while in the Polar 
seas, saw the inverted image of his father's ship, although it was then really 
below the horizon, and thirty miles distant. This phenomenon, like the former, 
may be explained by the refraction of the rays of light through atmospheric strata 
of very different densities. 

Aberration. 

Note 45, Page 172. 
Although the discovery of aberration might have been made without observa- 
tion, yet the fact was found out before it was seen, as a consequence of reasoning. 
It seems clear, that since light and the spectator on the Earth are both in mo- 
tion, the apparent direction of an object will be determined by a composition of 
these motions. The aberration of light, the greatest astronomical discovery of 
the eighteenth century, belongs to Bradley, then Professor of Astronomy at 
Oxford, and afterwards Astronomer Royal at Greenwich. Bradley and Moly- 
neux, in 1725, were making observations in order to discover the existence of the 
annual parallax of the fixed stars ; but Bradley found a minute apparent motion 
which the annual parallax could not produce. In 1727, he resumed his observa- 
tions with a new instrument, but again without success ; for he was still at a loss 
to account for this apparent motion. At last accident favored him, by turning 
his inquiries into the right channel. Being one day in a boat on the Thames, he 
observed that the vane on the top of the mast indicated a different direction for 
the wind as the boat changed her course. Here he found a solution of his pro- 
blem. The boat represented the Earth moving in different directions in different 
seasons ; the wind was the light of a star. Thus the phenomenon was found to 
be an optical illusion, occasioned by a combination of the motion of light with the 

* Phil. Trans. 1798, vol. Ixxx-viii. 



NOTES. • 397 

motion of his telescope. He now traced the consequences of this idea, and found 
the answer to his inquiries. In 1729 he communicated his discovery to the Royal 
Society. His paper is a very happy narrative of his labors. His theory was so 
sound that it has never been contested, and his observations so accurate that 
the quantity which he assigned as the maximum of change, amounting to one- 
ninetieth of a degree in declination, has scarcely been altered by more recent astro- 
nomers. The aberration of light also affords a direct proof of the motion of the 
Earth in its orbit. 

Proper Motion. 

Note 46, Page 172. 

The idea of a proper motion of the stars was vaguely expressed by Lucretius ; 
and it was thought that but for such a motion all the bodies belonging to our sys- 
tem would fall together, and form one chaotic mass. 

Halley conceived the whole solar system, with the stars, to be in motion round 
some unknown body, the centre of gravity of the grand whole. But what that 
great centre is, or where it is situated, is yet a mystery. 

" As God in heaven 
Is centre, yet extends to all." — Mn,TON. 

Halley's theory as to the proper motion of the stars has never fallen to the 
ground, but has been maintained by Le Monnier, Cassini, and others. Tobias 
Mayer, a German astronomer, who lived about the middle of the eighteenth cen- 
tury, made numerous observations, all tending to confirm the fact. He thought 
that the changes observed might be accounted for by a progressive motion towards 
a certain quarter of the heavens. John Henry Lambert, one of the ablest geome- 
tricians of the eighteenth century, in his Cosmologische Briefe, (1761,) suggested 
that the stars in the universe are collected into systems, that all these systems are 
in motion, and that the individual stars of each system move round a common 
centre of gravity ; that all the systems of the universe, as a whole, revolve about 
some grand centre as a grand whole, which alone of all the creation can be in a 
state of absolute rest. 

The possibility of the motion of our Sun in space was treated on theoretical 
principles by Dr. Wilson, of Glasgow. M. de Lalande deduced the same opinion 
from the rotary motion of the Sun, supposing that the same force which causes it 
Jo revolve on its axis would also give it a motion of translation in space. 

Sir William Herschel, after a variety of observations and calculations, which he 
transmitted to the Royal Society, remarks, that from the time the proper motion 
of the stars was first suspected by Halley, we have had continued observations to 
show that Arcturus, Sirius, Aldebaran, Procyon, Castor, Rigel, Altair, and many 
more, are actually in motion ; and considering the short time during which we 
have had observations accurate enough for the purpose, it may rather be won- 
dered at that we have already been able to find the motions of so many, than 
that we have not succeeded in discovering the like alterations in moi-e. Besides, 
we are well prepared to find numbers of them apparently at rest, as, on account 
of their immense distance, a change of place cannot be expected to become visible 



398 



BOUVIER S FAMILIAR ASTRONOMY. 



to us till after thousands of years of careful attention and close observation, 
admitting every one of them to have a progressive motion. This consideration 
alone would lead us strongly to suspect that there is not, strictly speaking, one 
fixed star in the heavens ; but many other facts will render this so obvious, that 
there can hardly remain a doubt of the general motion of all the starry systems, 
and consequently of the solar one among the rest. See Phil. Trans. 1783, vol. Ixxiii. 



Parallax. 

Note 47, Page 175. 

The most casual observer will see that the Sun, the Moon, and those celestial 
bodies which we call planets, have each a particular path, which does not occupy 
the same point in the heavens for any two consecutive days. If the Moon should 
be near a bright star this evening, to-morrow its relative situation will be changed ; 
it will have moved, and the star will have retained its position with respect to the 
other bright stars near it, proving conclusively that the Moon is constantly chang- 
ing its position in the space which separates it from the stars. The same observa- 
tions will hold good with the Sun and the planets. It is known that the volume 
of the Sun is nearly 1,400,000 times that of the Earth, that Jupiter is nearly 
500,000,000 of miles from the Sun, &c. These assertions are astounding to those 
not conversant with the powers of the geometrician to measure inaccessible dis- 
tances ; but that subject may be elucidated by a figure. 

It is well known that the sum of the three angles of any triangle are equal to two 

right angles, or 180°. 

Pig. 219. 




Let us imagine an astronomer situated at A on the Earth. A C (fig. 219) should 
measure the angle B A Z, or the distance of the celestial body situated at B from 
the zenith Z, which is the complement of the angle BAH, (the height of B from 
the horizon at A,) or, what is the same thing, the difference between B A H and 
90°. He will find that the angle B A C is a supplement to the angb B A Z, or 
that the angle B A C is 180°, minus the angle B A Z. 



NOTES. 399 

If, at the same time, a second observer, situated at C, the centre of the Earth, 
measure the angle B C Z, the distance of B from the zenith Z, then in the triangle 
ABC will be found the three elements necessary to find the distance B C of the 
body B from the centre C of the Earth. 

Thus the distance of B from C may be found by a simple trigonometrical pro- 
cess, although the point B be inaccessible ; for, by observing B from the two extre- 
mities of the line A C, the length of which is known, and noting the angle it makes 
with each end of this line, its distance may be ascertained. 

From the points A and C the star B is referred to a and c among the stars, and 
this celestial arc a c measures the angle B, which is called the parallax of the 
body B : thus, the parallax of the Sun or a planet is the angle, under which 
a spectator situated on that body would see the radius A C of the Earth ; it is 
also the arc a c of the apparent displacement of the body as seen from the point 
A and the centre C of the Earth. 

When the body is situated at B'' on the horizon of the spectator at A, the pa- 
rallax A B'' C is said to be horizontal, which is the greatest parallax a star can have, 
situated at the distance B C or B^ C when seen in the horizon ; for it may easily 
be seen that as the body rises towards the zenith, the lines A B and C B coincide 
more and more nearly with C Z, until finally they are united. Thus, the parallax 
of a body decreases as it approaches the zenith, at which point it vanishes alto- 
gether. 

When the Moon is in the horizon at the moment of rising or setting, an imagi- 
nary line drawn from her centre to the eye of the spectator, and another drawn 
to the centre of the Earth, would form a right-angled triangle with the Earth's 
radius. Now, the radius of the Earth is known to be 3958 miles; and as the angle 
at the Moon can be measured, all the angles and one side of the tri'angle being given, 
the Moon's distance from the Earth's centre may be found by the rules of trigono- 
metry. Simultaneous observations were made at Berlin and the Cape of Good 
Hope on the zenith distance of the Moon at the time of her passage of the meri- 
dian on a certain day, from whence her mean horizontal parallax was found to be 
3454''^ -2, which gives the mean distance of the Moon from us equal to sixty semi- 
diameters of the Earth, or 240,000 miles, nearly. 

This method, however, is not sufficiently accurate for finding the distance of a 
body so remote as the Sun. In order to discover the distance of our central 
luminary, observations of the transits of Venus have been accurately taken. When 
that planet is in, or within \\° of, her nodes, she is sometimes seen to pass over 
the Sun's disc like a black spot. If the semi-diameter of the Earth had no sensi- 
ble magnitude at the Sun or Venus, the line described by the planet on the Sun's 
disc and the duration of the transit would be the same wherever it would be seen 
from the Earth; but as the Earth's radius has a sensible magnitude as seen from 
the Sun, the line described by Venus in her passage over the Sun's disc varies in 
position according to the station of the observer. As the line described by the 
planet varies according to the position of the observer, so does the duration of the 
transit, These differences of time are entirely owing to parallax. The transit 
of 1769 was observed in different parts of the Earth ; that at Otahoite was the 
object of Captain Cook's first voyage. From the observations made at that time, 



400 bouvier's familiar astronomy. 

the Sun's parallax was found to be about 8^^ -6, which gives nearly 96,000,000 of 
miles as the mean distance of the Earth from the Sun. 

The parallax of the fixed stars is found by assuming the diameter of the Earth's 
orbit as a base line, and even this enormous extent is too minute to subtend any 
angle at the distance of most of the fixed stars, comparatively few having been 
found to have any sensible parallax. As far as our observations have extended, 
the star «t Centauri has a gf eater parallax than any other ; yet its distance is such 
that the orbit of the Earth, when seen from it, appears like a mere point. Thus, 
at the nearest fixed star, our whole system of Sun and planets, if viewed through 
a good telescope, would appear so small that it might be covered by a spider's 
thread. 

Distances of the Stars. 

Note 48, Page 177. 

The distances of the fixed stars have hitherto been considered too immense for 
the astronomer ever to hope of measuring or calculating with even an approxima- 
tion to the truth. The diameter of the Earth was found to be too minute a base 
for such immense triangles as those required to measure the bounds of our solar 
system; therefore all hope of estimating the distances of the stars was for a 
time extinct. But by enlarging the base, by substituting the vast diameter of the 
Earth's orbit, it was thought the problem might easily be solved. Even this dis- 
tance of 190,000,000 of miles was found an insufficient base, and, until very 
recently, defied every refinement of modern observation. 

Piazzi, a celebrated Italian astronomer, and, after him. Professor Bessel, a Prus- 
sian astronomer, discovered an unusually great proper motion in the star marked 
No. 61, in the constellation Cygnus, according to Flamstead's catalogue of stars. 
This star is double when viewed through a good telescope, and observation has 
assigned a period of about five centuries for the one star to revolve round the 
other. Besides this, they are travelling together through space at the incredible 
velocity of six thousand millions of miles in an hour. Yet, at the immense dis- 
tance which it is situated from us, the path which that star pursues appears to 
be a straight line. 

The observations on 61 Cygni were continued diligently for three or four years, 
and the reward of those labors was, the discovery of a parallax of a little less than 
one-third of a second. An arc of one-third of a second is equal to the 657,700th 
part of the radius. Thus, if the distance of a star be the radius of a circle of 
which the distance of the Sun from the Earth, or 95,000,000 of miles, is an arc 
measuring only one-third of a second, — which, as before stated, is equal to the 
657,700th part of that radius, — it follows, that the distance from the Sun to the 
star must be 657,700 times the distance of the Sun from the Earth ; which would 
equal 58,880,525,000,000 of miles, an expanse far beyond the powers of human 
conception. Light, which travels at the rate of nearly 200,000 miles in a second 
of time, would require more than nine years to accomplish the journey. 

The electric fluid travels at the mean rate of 20,000 miles in a second under 
ordinary circumstances ; therefore, if it were possible to establish a telegraphic 
communication with the star 61 Cygni, it would require ninety years to send a 
message there. 



NOTES. 401 

For the very nice and accurate researches of Professor Bessel, the Royal As- 
tronomical Society of London awarded him a gold medal — "Researches," says 
Sir John Herschel, "which have gone far to establish the existence, and to mea- 
sure the quantity, of a periodical fluctuation, annual in its period and identical 
in its law with parallax. 

The parallax of * Centauri, one of the brightest stars in the southern hemi- 
sphere, amounts to a second of space ; consequently it is nearer to the Earth than 
any star that is known. Professor Henderson and Mr. Maclear have fully con- 
firmed the annual parallax of this star to amount to a second of arc, conse- 
quently it is not so distant as 61 Cygni ; for a parallax of one second gives about 
twenty billions of miles as its distance from our system ; a ray of light would 
arrive from a Centauri to us in a little more than three years, and a telegraphic 
despatch would arrive there in thirty years. 

The Milky Way. 

NoTK 49, Page 181. 

It has been found that by increasing the power of our telescopes, more and 
more stars are brought into view, verifying Halley's supposition that the number 
of the stars is so great, that if we cannot call it infinite, it is too vast for a finite 
number to represent. Their distances, too, from each other, are only to be ex- 
pressed by infinite numbers. These nocturnal suns are beyond expression innu- 
merable, and the most ardent imagination fails to set any bounds to the extent 
of the universe. 

Our Sun is supposed to be one individual star, forming only a unit in a cluster 
of many millions, and occupying a place among those minute bodies whose com- 
bined light forms the Milky Way, which is itself a ring, of which there are thou- 
sands to be seen in the realms of space by the aid of telescopes. These rings or 
clusters are composed of suns like ours, each one of which, doubtless, forms the 
centre of a planetary system. And if our Milky Way or cluster of stars be viewed 
from some of these other milky ways or distant clusters, it would no doubt ap- 
pear like a small collection of whitish, cloudy light, being too distant to be resolved 
into stars. Such is the immensity of the creation, of which, perhaps, we see but 
a minute part of the grand whole ! 

Sir William Herschel supposes the stars to be scattered over an indefinite por- 
tion of space in such a manner as to be almost equally distributed. Those stars 
of the first magnitude he assumes to be the nearest to our system ; those of the 
second magnitude, at d(iuble the distance from us ; those of the third magnitude, 
at three times the distance, and so on. Then supposing a star of the seventh 
magnitude to be seven times as distant as a star of the first magnitude, it follows 
that an observer enclosed in a globular cluster of stars, and not far from its 
centre, could not be able to see the boundary of the cluster by the naked eye. If 
he should be enclosed in a ring or annulus, he might, through the opening of the 
ring, see neighboring clusters, which, to him, would seem like patches of faint, 
whitish cloud. His own zone or milky way would appear to surround the heavens, 
which would be studded with stars of all degrees of brilliancy. In such a situa- 

26 



402 bouvier's familiar astronomy. 

tion he would naturally conclude that these stars, of which he cannot see the ten- 
thousandth part, must comprise the whole extent of the univei'se. Now, give 
him the telescope, and soon he begins to suspect that the milky zone is composed 
of stars whose individual lustre could not be appreciated by the naked eye. He 
now discovers numberless nebulous patches, apparently components of his own 
system, which to him comprehends every celestial object. By increasing his tele- 
scopic power, he finds these nebulous patches to consist of innumerable stars, 
while other nebulous patches of milky light are revealed. These in their turn are 
resolved into stars by the use of higher telescopic power; and the astonished 
observer finds that he has only just set out on his voyage of discovery ; that in 
distant regions there are thousands of milky ways of even more stupendous extent 
than his own, although that may boast of having a diameter of hundreds of billions 
of miles. Should he now be armed with a giant telescope, like that of Lord Rosse, 
and in the profundity of space find a cluster of thousands of stars whose distance 
is barely within the penetrating power of his instrument, he would have a range 
of vision of no less than 11,765,475,948,678,678,679, or eleven trillions, seven 
hundred and sixty-five thousand four hundred and seventy-five billions, nine hun- 
dred and forty-eight thousand six hundred and seventy-eight millions, six hundred 
and seventy-eight thousand, six hundred and seventy-nine miles ! ! ! 

New Star-in Cassiopeia. 

Note 50, Page 186. 

The new star in Cassiopeia, which appeared in 1572, and by some called Tycho 
Brah^'s star, was first seen at Wittemburg, by Schuler, in August of that year ; 
by Hainzel, at Augsburg, on the 7th, and by Cornelius Gemma on the 9th, of 
November. But as there is no reason to suppose there was in those days any 
communication between the observers, the discovery of each may be considered as 
independent of the other. It was seen by Tycho Brahe on the 11th of November, 
and attracted a great deal of attention among the learned as well as the illiterate. 

It increased in brilliancy rapidly, until it surpassed Sirius in splendor, and 
became visible in the daytime : thus it was watched continually. It did not 
retain this great splendor for a long time, but gradually decreased in brilliancy 
till March, 1574, when it disappeared. 

Tycho Brahe had studied chemistry and astronomy in Copenhagen and Leipsic, 
and on his return to Denmark, in 1571, on account of his superior attain- 
ments, was invited by the king to his court. Tycho had just fitted out apart- 
ments for an observatory and laboratory, in which he ^ent much of his time. 
His observatory was furnished with the best instruments of the times, and was 
the object of much curiosity. It was the following year, 1572, after Tycho's 
instruments were adjusted, that this brilliant star appeared. A writer of the 
time says : " By a strange instinct of Providence, wero those admirable instru- 
ments made and erected by Tycho a little before the appearing of this starre, as 
if either the starre had stayed for his tooles, or he had foreseen the birth of that 
starre." Tycho's attention had been turned to alchymy, by which he expected to 
turn the baser metals into gold ; but the sudden appearance of the brilliant star 



NOTES, 403 

dissipated all those visions, and roused him to active exertions in his observatory. 
He made tolerably correct observations of the place and magnitude of the new 
star, but at first refused to publish them, saying that he considered it *'a disgrace 
for a nobleman either to study such subjects or communicate them to the public." 

Tycho conjectured that this new star might have been formed by a sudden con- 
densation of nebulous matter ; and since then M. Arago has advocated the same 
opinion.^ Professor Stephen Alexander, of Princeton, New Jersey, considers the 
facts as consistent with supposed changes of density of the mass, and that the 
material which had been rapidly condensed would, by increased rapidity of rota- 
tion, be soon dispersed in all directions round the centre. It is not a little curious, 
moreover, that the changes in color which were observed are not, upon the whole, 
inconsistent with the supposed changes of density and distribution of material, f 

If, however, this star should be identical with those which appeared in 945 and 
1264, it must be periodical or variable. Sir John Herschel thinks it possible it 
may appear again about the year 1872. Dr. Dee, an English divine of great 
learning, and an astrologer, suggested the idea that the new star might move in a 
direct line to and from the Earth, which he thought might account for its sudden 
appearance and disappearance. 

By the astrologers of the time the appearance of the new star was considered 
as portending some great event. This star was first white, then yellow, after- 
wards red, and finally bluish or grayish, which induced La Place to suppose that 
its changes were owing to the action of fire. Mrs. Somerville says that "it is 
impossible to imagine any thing more tremendous than a conflagration that could 
be visible at such a distance." But Captain Smyth, in a conversation with this 
learned lady, says that she was not at all inclined to grant that so vast a confla- 
gration was within the limits of probability. 



Meteorites and Aerolites. 

Note 51, Page 197. 

All unusual appearances in the heavens, such as comets, auroras, &c., were by 
the ancients classed together as meteors. But meteors appear in the clear sky, 
dart along thi-ough the heavens, and generally disappear without noise, and with- 
out the fall of any residuum or of any tangible substance whatever; whereas 
meteorites usually explode with a loud noise, which is often attended by the fall 
of stones greatly heated, and incrusted by* a shining, black substance. 

Great falls of meteoric stones are recorded in the Chinese annals as early as 
B. c. 700 ; and Aristotle, who lived 300 years before our era, supposed the same 
phenomenon to be caused by fragments of rocks having been raised by whirlwinds, 
and after remaining for a time in the regions of space, have again found their way 
to our Earth. Showers of stones are mentioned by both Pliny and Livy. Some 
of the Greek philosophers attributed to them a solar origin. Pliny says that 
Anaxagoras, an Ionian philosopher, who lived in the fifth century before Christy 

* See Annuaire, pour 1842, pp. 432, 433 Ddambre, Hiiloire de F Astronomic Modernc 
t American Astronomical Journal, vol. ii. p. 149. 



404 bouvier's familiar astronomy. 

even predicted a fall of aerolites fi-om the Sun. Diogenes Laertius entertained the 
opinion that the enormous meteorite which fell at iEgos Potamos, b. c. 465, had 
been projected from the Sun. 

As meteorites appear to have a common origin, many philosophers have supposed 
them to be fragments of the rocks of the Moon ejected by lunar volcanoes ; those 
luminous spots on the unilluminated part of the Moon's disc having been ascribed 
to light emitted by burning mountains. The opinion that meteorites are frag- 
ments thrown from lunar craters was first entertained by Terzago, an Italian phi- 
losopher, in the year 1660, on the occasion of the fall of a meteorite at Milan, 
which killed a Franciscan monk. But the velocity of projection must be equal to 
more than 20 miles in a second in order to throw these masses of stone beyond 
the influence of the Moon's attraction, whereas the projectile force of any terres- 
trial volcano is not equal to one mile in a second ; therefore it seems highly impro- 
bable that meteorites can be of lunar origin. 

Some remarkable obscurations of the Sun are said to have happened, which, if 
true, the causes would be difl&cult to explain. For instance, in the year 1090, 
the Sun was obscured for three hours, and in 1203 during six hours, although the 
sky was cloudless. In 1547, an obscuration of the Sun occurred at midday, with 
a clear sky, and continued for three days, during which time the stars were always 
visible. Kepler attributed this phenomenon to a materia cometica, or to a black 
cloud formed by certain materk,! exhalations from the Sun itself.* The obscura- 
tions of 1090 and 1203 were thought by Chladni to be owing to the passage of 
meteoric masses between the Sun and Earth, causing an eclipse of that luminary. 

Meteorites are now supposed to have a cosmical origin, and to belong to a 
group or annulus which the Earth encounters in her orbit. The obliquity of their 
paths in their descent to the Earth, and the explosions attendant on their fall, 
seem to favor that doctrine. 

In 1606, Halley observed a fire-ball, which he pronounced to be of cosmical 
origin ; but it was not till 1794 that Chladni recognised a connection between fire- 
balls and meteorites. 

It is only when meteorites arrive within the limits of our atmosphere and ignite, 
that we have any knowledge of their existence ; for they are only visible when 
they become luminous by ignition. They are by some supposed to be fragments 
of asteroids, or small bodies, of which there are probably myriads throughout our 
system, all revolving round the Sun as a common centre, and which are sometimes 
diverted from their orbits by some disturbing force. Portions of these asteroids 
may be attracted to other planets as well as to our Earth. 

The analysis, by Mr. Faraday, of a very large meteorite found at Cape Colony, 
affords a view of the general nature of their constituents : Silica, 28-9 ; protoxide 
of iron, 33-22; magnesia, 19 '2 ; alumina, 5-22 ; lime, 1-61; oxide of nickel, 0-82; 
sesquioxide of chromium, 0-7; sulphur, 4*24; water, 6*5. The average specific 
gravity of meteorites equals 3-0. 

The falls of meteoric stones have been found, from numerous observations, to 
be much fewer in December and January than in June and July. If, therefore, 

* Cosmos, vol. i. p. 121. 



NOTES. 405 

tlie meteorites belong to that system of asteroids which lie between Mars and 
Jupiter, the Earth would be more likely to meet with them when at her aphelion, 
or farthest distance from the Sun, which is in June and July, than in December, 
when she is nearest to the Sun. 

Le Verrier has made a calculation which shows that the mean mass or system 
of the asteroids cannot exceed one-fourth of the mass of the Earth ; and also that 
it is highly probable that the system of asteroids is at its perihelion when the Earth 
is at aphelion; consequently, the Earth is nearer to them at that period (June) 
than any other time of the year. This theory is favorable to the supposition that 
meteorites are outliers, or minute accompaniments, to the system of asteroids. 
Their specific gravity is supposed to be about the same as that of the asteroids. 
See Am. Jour. Science and Arts, Jan. 1855. 

^ Zodiacal Light. 

Note 52, Page 198. 

The phenomenon of zodiacal light is not mentioned by any of the early writers, 
although it certainly could not have escaped the notice of the Arabian and Gre- 
cian astronomers. 

An ancient Aztec MS. preserved in the Biblioth^que du Roi, at Paris, contains 
an account of a remarkable light which was visible for forty consecutive nights in 
the year 1509, and which Montezuma regarded as an omen of his downfall. This 
is now supposed to have been an unusually luminous zodiacal light, as it is 
recorded to have been seen in the immediate vicinity of the Sun when he was 
below the horizon. 

This phenomenon was first observed in Europe by Childrey, in 1661, the descrip- 
tion of which occurs in his Britannia Baconica. Dominic Cassini, in 1663, was, 
however, the first who investigated all the phenomena attendant on zodiacal light. 

Humboldt observed the various intensities of this light in the tropical regions ; 
he has seen it flicker and wane from a very strong to a pale light, and then sud- 
denly shoot up again as bright as before. 

A phenomenon, similar to the zodiacal light, if not identical with it, is described 
by John Christopher Sturmius, a German philosopher, who flourished during the 
last half of the seventeenth century. He called it Ileliocometes, or comet of the 
Sun, because the light which he sometimes observed at sunset appeared like a 
lai'ge column of light attached to the Sun, like a tail to a comet. 

The Rev. Geo. Jones, of the U. S. N., a member of the corps belonging to the 
Japan Expedition, has made numerous observations on the zodiacal light, showing 
that in that climate "it never fails to be seen when the Moon and clouds do not 
interfere." See United States Exploring Expedition, vol. v. p. 477. 

Biot inferred that the zodiacal light might be in some way connected with the 
Earth. By experiments he found that it gave more heat than the tail of a comet. 

When at the summer solstice, according to the observations of the Rev. Mr. 
Jones, the zodiacal light was visible from 11 till 1 in both horizons, with their 
apices approaching each other. 

From the facts which he accumulated with regard to this singular phenomenon, 



406 bouvier's familiar astronomy. 

Mr. Jones concludes this light to be a nebulous ring surrounding the Earth, in 
■which the meteors and even the meteorites may have their origin. Observations 
show there is a constant commotion ■within the ring itself, from -which the nebu- 
lous matter half agglomerated in different parts of it may be thro^wn beyond its 
sphere, and fall to the Earth by its superior attraction. 

Professor Peirce concurred -with Mr. Jones in his theory, and Professor Gould 
thought that Mr. Jones's observations threiv more light on the nature of zodiacal 
light than all before him. — Proceedings of the American Association for the Advance- 
ment of Science, August, 1855. 



Origin of the Constellations. 

Note 53, Page 200. 

Those -who hare made Oriental literature their study are united in the opinion 
that the constellations were not grouped in reference to any resemblance certain 
collections of stars may have to the forms of animals or monsters, but that they 
■were intended to indicate the successive returns of the seasons, and the various 
labors of the husbandman. 

The zodiacal constellations, ■which are supposed to be of far more ancient origin 
than the northern or southern, appear to represent a picture of the whole year, 
considered in connection with the sunlight, vegetation, and agricultural labors. 
The names of the months in some countries of Egypt and the East, of which the 
celestial signs were the hieroglyphic expression, were given in accordance with 
the state of vegetation, &c. Even in modern times the French calendar under the 
Republic adopted the same plan; thus Germinal was blossom month, Fructidor 
was flower month, &c. The Germans in some districts call June the hay month, 
July, the harvest month, &c. 

Various nations have claimed the honor of the construction of the constella- 
tions, especially the Chaldeans, the Egyptians, the Chinese, and the nations of 
India, all of whom assert that the science of astronomy had its origin with the 
founders of their several empires. Although the various groups of stars bear 
little or no resemblance to the figures in which they are included, yet the simi- 
larity of these configurations, as adopted by different nations, bespeaks a common 
origin. Thus the Chaldean, Egyptian, Indian, Arabic, and Grecian zodiacs are 
very similar to each other ; and if it be true that the various divisions or signs 
were formed in reference to the position of the Sun and the employments of the 
agriculturist, the Egyptian origin is in some respects the most probable one.^^ 

Still a difficulty presents itself, if we suppose the constellations of the zodiac to 
have been named by the Chaldeans, Egyptians, or Ethiopians, as the seasons in 
these countries do not coincide with the symbols. But if we follow the opinions 
of some Orientalists, and take the position of the heavens as it was about 15,000 
years ago, the coincidence will be complete. In that case the zodiac would be a 
true representation of the heavens and of the seasons of Egypt at that epoch. 



* Lucian assigns Ethiopia as the birthplace of astronomical science, from whence it descended to 
the Egyptians. — Lucian, Astrdl. p. 985. 



NOTES. 407 

Hence it is inferred that the figures traced on our zodiac are not fanciful, designed 
at hazard and without any object, but that they are the natural symbols of the 
return of the different seasons among the people with whom they originated. 

If we take the position of the sphere as it was at the epoch above mentioned, 
we shall find Capricorn to be the summer solstitial point, the reverse of which 
holds at present.* Capricorn is represented as amphibious, having the head of a 
goat and the tail of a fish ; the next sign, Aquarius, is represented as a man pour- 
ing water from an urn, which finally forms a river; the third sign, Pisces, is 
composed of two fishes united by a cord. These three signs are symbolical of the 
situation of Egypt during the three months which follow the summer solstice ; 
for it is well known that soon after that time the Nile inundates the country 
through which it flows, and does not completely recede until the autumnal equi- 
nox. Thus is the overflow of the Nile typified by the three signs, which evidently 
relate to water — namely, the goat with a fish's tail, the urn from which the river 
issues, and the fishes. A friend of the author's saw a small ancient vase in the 
possession of the Earl of Warwick, with an inscription on it, of which the follow- 
ing is a translation : 

" Wise ancients knew, when Orater rose to sight, 
Nile's fertile deluge had attained its height." 

The greatest northern declination, or highest elevation of the Sun, is typified 
by the Goat, which always delights to browze on the summits of mountains or 
the peaks of the highest rocks. 

The Earth is soon covered with rich pasturage after the water has retired, 
which is represented by the Ram, the chief or head of the flock. The commence- 
ment of agricultural labor is represented by the Bull, the sign the Sun occupied 
at that time of year at the period above mentioned. Manilius speaks of the Bull 
as an emblem of rural labor. The appearance of the grain above the ground on 
the following month is typified by the Twins, two very young children, or, accord- 
ing to some of the Oriental spheres, by two young goats. 

The Sun next arrives at Cancer, which was, at the remote date above stated, 
the winter solstitial point; at which time he retraces his path, which is aptly 
typified by the retrograde movements of the Crab. One month after this the har- 
vest is ripening for the sickle, which is represented by the Lion, as symbolical of 
the strength which vegetation has already acquired. According to Diodorus, the 
time between seed-time and harvest in Egypt is only about four months ; so that 
the sign for the month of harvest would be Virgo, which is represented by a 
female with an ear of wheat in her hand, signifying the time of harvest. The 
Persians call this sign the "ear of wheat," from whence is derived Spica, the 
name of the principal star in this constellation. The sign of the Scales or Balance, 
which follows Virgo, announces as important an epoch in the astronomical year 
as that of the ear of wheat in the rural year. The Balance signifies the equality 

* Father Kircher, a German Jesuit, and Professor of Hebrew and Mathematics, who died at Rome 
in 1680, was a man of great learning and untiring diligence. His favorite study was Egyptian 
hieroglyphics; and in a work on that subject gives the representation of an Egyptian Planisphere, 
which places Capricorn at the summer solstice. 



408 bouvier's familiar astronomy. 

of the days and nights — the equal division of light and darkness. This sign at 
that time answered to the vernal equinox. The supposition that the Balance was 
originally placed at the autumnal equinox becomes chimerical, when it is remem- 
bered that the science of astronomy had been known long before the asterisms of 
the Balance could answer to the autumnal equinox. 

The Scorpion, being a venomous reptile, is emblematical of the sickly season. 
Sagittarius, the Archer, in some of the ancient zodiacs is only represented by a 
bow and arrow. As the arrow, which is just ready to leave the bow, is an 
emblem of swiftness, it represents the season of high winds. 

In the Philosophical Transactions for 1772, there is a paper containing a com- 
munication from Nevil Maskelyne, then Astronomer Royal, in which there is a 
description, together with an engraving, of an Indian zodiac, in which Cancer is 
represented on the south and Capricorn on the north side* of the ceiling. The 
signs are the same, or nearly the same, as those now in use. 

The foregoing is among the many conjectures which have been made with 
regard to the origin of the zodiacal constellations, and which, though highly inge- 
nious and very plausible, is still open to objections. 

As to the origin of the other asterisms, we have no reason to suppose it is en- 
titled to as high antiquity as that of the zodiacal constellations, though Homer 
speaks of the Great Bear, Bootes, Orion, and Sirius. The Pleiades, Orion, Arc- 
turus, and Sirius are mentioned by Hesiod. The three first are also spoken of in 
Holy Writ. 

Sir John Herschel is of opinion that there was no object in thus naming the 
constellations. In treating of that subject, he says : " The constellations seem to 
have been almost purposely named and delineated to cause as much confusion and 
inconvenience as possible. Innumerable snakes twine through long and contorted 
areas of the heavens, where no memory can follow them ; bears, lions, and fishes, 
large and small, northern and southern, confuse all nomenclature. A better 
system of constellations might have been a material help as an artificial memory." 

But the present stellar arrangement, at least as regards the old forty-eight 
asterisms, is considered by some modern astronomers as highly useful for those 
who are not supplied with instruments for finding the places of the fixed stars. 
By means of this grouping into constellations, the eye becomes familiar with the 
principal stars of each group or family, and thereby aids the memory in retaining 
the places of others less conspicuous. " Those," says Smyth, " who would sweep 
away the constellations altogether, as incongruous absurdities or wicked pagan 
allusions, seem rather reckless about the consequences of such a measure on 
astronomical history, chronology, and extra-observatorial practice." 

It is evident from what has been said that the origin of the constellations, espe- 
cially those of the zodiac, is wrapped in the obscurity of the most remote anti- 
quity, from which it would now be impossible to wrest it. 

* The sketch is by Mr. J. Call, who copied it from a pagoda near Cape Comorin, in India; a similar 
one was found by the same author on the ceiling of a temple near Mindurah. — Phil. Trans, vol. Ixii 
n. 353; also Mrs. SomerviRes Phys. Sci. p. 83, &c. 



NOTES. 



409 



The Pole Star four thousand years ago. 

Note 54, Page 217. 

The following, from Sir John Herschel's Outlines of Astronomy, shows the 
changes in the celestial pole in four thousand years : 

" At the date of the erection of the Great Pyramid of Gizeh, which precedes the 
present epoch by nearly 4000 years, the longitudes of all the stars were less by 
55° 45'' than at present. Calculating, from this datum, the place of the pole of 
the heavens among the stars, it will be found to fall near a Draconis ; its distance 
from that star being 3° 44'' 25''^. This being the most conspicuous star in the 
immediate neighborhood, was therefore the Pole Star at that epoch. The latitude 
of Gizeh being just 30° north, and consequently the altitude of the North Pole 
there also 30°, it follows that the star in question must have had, at its lower cul- 
mination at Gizeh, an altitude of 26° 15'' 35^''. Now it is a remarkable fact, 
that of the nine pyramids still existing at Gizeh, six (including all the largest) 
have the narrow passages, by which alone they can be entered, (all which open 
out on the northern faces of their respective pyramids,) inclined to the horizon 
downwards at an angle as follows : 

1. Pyramid of Cheops 26° 41' 

2. " " Cephren 25 55 

3. " " Mycerinus 26 2 

4 27 

5 27 12 

9 28 

Mean 26° 47' 

At the bottom of every one of these passages, therefore, the Pole Star must 
have been visible at its lower culmination, a circumstance which can hardly be 
supposed to have been unintentional, and was doubtless connected (perhaps super- 
stitiously) with the astronomical observations of that star, of whose proximity to 
the pole at the epoch of the erection of these wonderful structures we are thus 
furnished with a monumental record of the most imperishable nature." — Hers. 
Ast, p. 174. 

Great Nebula of Canes Venatici. 

Note 55, Page 219. 

This wonderful object was discovered in the year 1772, by Messier, a noted 
French astronomer, and described by him as faint double nebulae, whose centres 
were 4'' 35'''' apart, but with the bodies in contact. 

"Supposing it," says Sir John Herschel, "to consist of stars, the appearance 
it would present to a spectator placed on a planet attendant on one of them, eccen- 
trically situated towards the north preceding quarter of the central mass, would be 
exactly similar to that of our Milky Way, traversing, in a manner precisely analo- 
gous, the firmament of large stars, into which the central cluster would be seen 
projected, and (owing to its greater distance) appearing, like it, to consist of stars 
much smaller than those in other parts of the heavens. Can it then be that we 
have here a brother system bearing a real physical resemblance to our own?" 



410 bouvier's familiar astronomy. 

Since Sir John Herschel formed his catalogue of northern stars, Lord Rosse's 
great telescope has revealed this nebula to be spiral, with two nuclei. But ^ill 
some astronomers trace a resemblance between it and the Milky Way, which is 
also supposed to be spiral.* From the above, it will be seen that the modern 
improvements in telescopes reveal more and more the wonderful structure of the 
universe, which, before their invention, had been a "sealed book." By their aid 
we are permitted to obtain a glimpse of that amazing display of the Omnipotent 
energy and power so fully manifested in the order and harmony of the innu- 
merable host composing the stellar universe. "We thus observe," says Lord 
Rosse, "that with each successive increase of optical power, the structure has 
become more complicated, and more unlike any thing which we could picture to 
ourselves as the result of any form of dynamical law of which we find a counter- 
part in our system. The connection of the companion with the greater nebula, 
of which there is not the least doubt, and in the way represented in the sketch, 
adds, as it appears to me, if possible, to the difficulty of forming any hypothesis. 
That such a system should exist without internal movement, seems to be in the 
highest degree improbable ; we may possibly aid our conceptions, by coupling 
with the idea of motion that of a resisting medium ; but we cannot regard such a 
system, in any way, as a case of mere statical equilibrium." — Phil. Trans. 1850, 
p. 504. 



* See The Origin, &c. of Clusters, Stars, and Nebulas, by Professor Stephen Alexander, American 
Astronomical Journal, toI. ii. p. 101. 



ASTRONOMICAL DICTIONAEY. 



Abbreviations. — Alt., Altitude ; A. M. Ante 
Ileridian, before noon, or Anno Mundi, 
the year of the world; App., Apparent; 
M., R. A., or a, Right Ascension ; B. A. C. 
British Association Catalogue; Barom., 
Barometer ; B. Z., Bessel's Zones; Cat., 
Catalogue; Circum., Circumference ; Col- 
lim., Collimator ; D., Direct; Dec. or d., 
Declination ; Deg., Degree ; Diam., Dia- 
meter; Diff., Difference; Dist., Distance; 
Ep., Epoch; Eq., Equator, ov Equinoctial ; 
Exter., External; Gr. M. T., Greenwich 
Mean Time ; Hor. Eq. Par., Horizontal 
Equatorial Parallax ; Inch, Inclination ; 
Inter,, Internal; M., Meridian, or Meri- 
dies, 3tidday, noon; Mag., Ilagnitude; 
Micr., Micrometer ; Min. or m.. Minute ; 
M. T., Mean Time ; N. L., North Latitude; 
N., North; Neb., Nehula; N. P. D., North 
Polar Distance ; n. p., north preceding ; 
n. f., north following ; N. S., New Style; 
Obs., Observation ; 0. S., Old Style; Par., 
Parallax; Vos., Position ; Tree., Preces- 
sion; 'K; Retrograde; Bad., Radius; Rev., 
Revolution; S., South; s. t, south folloic- 
ing ; s. p., south preceding ; S. L., South 
Latitude ; Sec. or s.. Second of time ; Su- 
pra Polum, ahove the pole; Sub polo., 
helow the pole ; Tang., Tangent ; Ther- 
mom., Thermometer, 

The following are found in almanacs : A. 
CQva.., Autumn commences; Aid., Aldeha- 
ran; Alg., Algenih ; Alt., Altair ; Ant., 
Antares ; Arc, Areturus ; Ari., Arietis ; 
Cap., Capella; Cas., Castor; Decl., De- 
clination; D. M., Days of the Month; 
D. W., Days of the Week; e., elongation; 
H.fEast; Fom., Fomalhaut; gr't, great- 
est ; gt. elong. or great, eg. W., greatest 
Western elongation ; great, bril., greatest 



brilliancy; H., Hour; H. L. N., Helio- 
centric Latitude North; Inf., Inferior ; 
in v., invisible; M., Morning, Month, or 
Minute; Mag., Magnitude; Mar., Mar- 
kab ; Pro., Procyon ; Beg., Regulus ; S. 
or © si.. Sun slow ; S. © fst.. Sun fast; 
S. com.. Summer commences ; Sir., Sirius; 
sup., superior ; W., Week or "West; "W. 
com.. Winter commences ; 7"*s., Pleiades. 
The following are the abbreviations used 
in catalogues for the names of observers : 

A. Argelander. 

B. Baily. 

B. Brisbane. 
Br. Briosche. 
D. Dawes. 
/\. Dunlop. 
JJI. Herschel, Senr. 
H. Herschel, Junr. 

h., followed by a number, refers to Sir 
J. Herschel's Catalogue of Northern 
Stars. 
H. and S. Herschel and South. 
M. Messier. 
P. Piazzi. 
S. South. 
2. Struve. 
Sm. Smyth. 
T. Taylor. 
The following abbreviations are used by 
Sir John Herschel in his catalogues : 
B. denotes bright. 

b. — brighter, 
br. — broad. 

c. — considerably. 
CI. or cl. — cluster, 
comp. — compressed. 

D. — Double. 

d. — diameter, distance. 

E. — extended,elongated,elliptic. 

411 



412 



BOUVIER S FAMILIAR ASTRONOMY. 



e. denotes extremely. 
ee. — excessively. 
F. — faint. 

f. — following, 
fig. — figure. 

g. — gradually. 
i. or irr. — irregular. 
/. — sweep. 

L. — large. 

1. — long or little. 

M. — in the middle. 

m. — much. 

N. — nebula. 

neb. — nebulous. 

n. — north. 

p. — pretty, preceding. 

pos. — angle of position. 

R. — round. 

r. — resolvable. 

S. — small. 

s. — south, suddenly. 

St. — stars. 

sc. — scattered 

V. — very. 

vv. — very exceedingly. 

Pise. Aust., Pisces Australia ; Cor. Aust., 
Corona Australis ; Triang, Austr., Trian- 
gulum Australis; Cat., Catalogue; At- 
mosp., Atmosphere. 
Aberration, {aherro, to wander from.) — 
The apparent displacement of the hea- 
venly bodies caused by the velocity of 
light, and the Earth's annual motion in 
her orbit. 
Aberration, Constant of. — During the in- 
terval which light requires to travel from 
the Sun to the Earth, the latter body has 
moved, with her average velocity, through 
an arc of 20 "45", which is therefore the 
amount of displacement in the Sun's 
longitude arising from the progressive 
motion of light ; this is called the Con- 
stant of Aberration, 
Aberration, Chromatic— The unequal re- 
frangibility of the rays composing white 
light, producing prismatic colors around 
the image of the object when viewed 
through a lens. 
Aberration, Diurnal. — A phenomenon 
caused by the movement of light com- 
bined with the rotation of the Earth on 
its axis. — Loomia. 



Aberration of the Fixed Stars.— The ap- 
parent annual motion of the fixed stars, 
produced by the velocity of light com- 
bined with the real motion of the Earth 
in its orbit, which causes each star appa- 
rently to describe a small ellipse, the 
centre of which is the true place of the 
star. 

Aberration of Planets and Comets.— An 
apparent displacement of their positions, 
arising from the progressive motion of 
light, whereby we always see them he- 
hind their true places in the heavens at 
the moment of observation. 

Aberration, Spherical. — The deviation of 
the rays of light from the true focus of a 
curved lens, producing a confused image 
of the object. 

Accelerated Motion. — Motion which re- 
ceives fresh accessions of velocity. 

Accelerating Force. — The force that acce- 
lerates the velocity of bodies. It is ex- 
pressed by the quotient arising from the 
motive or absolute force divided by the 
mass or weight of the body that is 
moved. — Sutton. 

Acceleration, (aceelero, to hasten.) — The 
increase of velocity in a moving body; 
the state of being quickened in motion. 

Acceleration of the Fixed Stars. — The 
time which the stars anticipate the Sun 
in a mean diurnal revolution, which is 
3w!. 55*9s. of mean time. In other words, 
it is the diflTerence of time between a 
sidereal and a solar day. 

Acceleration of the Moon. — A secular va- 
riation in the Moon's motion, amounting 
to about 11" in a century, occasioned by 
the variation in the eccentricity in the 
Earth's orbit. 

Accidental Colors.— Those colors which de- 
pend upon the affections of the eye, in 
contradistinction to such as belong to the 
light itself. These are also called com- 
plementary colors. 

Achemar. — A star of the first magnitude 
in the constellation Eridanus. 

Achromatic, (a, deprived of, and xp^t^Of 
color.) — Without color. A term first used 
by M. de Lalande to denote telescopes 
contrived to remedy aberrations and 
colors. 



ASTRONOMICAL DICTIONARY. 



413 



Achronical, (axpog, the extremity, and vol, 
night.) — A star is said to rise or set 
achronically when it rises or sets oppo- 
site to the Sun. 

Acolyte. — An attendant star, a companion 
star. 

Acmx. — A name given to the principal 
star in the Southern Cross, known as a 
Crucis. 

Acubens. — A star of the fourth magnitude 
in the constellation Cancer, known also 
as a Cancri. 

Acute Angle. — See Angle. 

Acute-angled Triangle. — A triangle hav- 
ing all its angles acute. 

Acute Cone. — A cone in which the vertical 
angle of the meridian triangle is less 
than 90°. 

Adara. — The name given to e Canis Ma- 
joris, a star in the constellation Canis 
Major. 

Adjacent Angles. — See Angle. 

Adjustment, {ad, to, and /ms^ms, just.) — The 
operation of bringing all the parts of an 
instrument into their proper relative po- 
sitions. 

Adjustment, Errors of.— Errors arising 
from an instrument not being properly 
placed, and from its movable parts not 
being properly disposed. 

Aerolite, (arjp, the air, and Aj0of, a stone.) — 
A name given to those mineral substances 
which sometimes fall from the upper re- 
gion of our atmosphere. They are also 
known by the names of meteoric stones, 
fire-balls, bolides, shooting-stars, meteor- 
olithes, &G. 

Afeichus. — An Arabic name for the con- 
stellation Ophiuchus. 

Age. — Used in chronology for a century. 

Age of the Moon. — The distance from her 
conjunction, or the number of days 
elapsed since the last new moon. 

Agena. — Another name for .8 Centauri, a 
star in the constellation Centaurus. 

Ain-al-thaur.— Another name for Aldeba- 
ran, which see. 

Air. — See Atmosphere. 

Akrab. — A name given to /? Scorpii, a star 
of the second magnitude in the constel- 
lation Scorpio. It is sometimes called 
Orajffias. 



Albireo. — One of the chief stars in the con- 
stellation Cygnus, known also as /? Cygni. 

Al Chiba. — An Arabic name for the star a 
Corvi, in the constellation Corvus. 

Alcor. — A small star in Ursa Major, known 
as p Ursae Majoris. 

Alcyone.— The principal star of the Plei- 
ades, called r] Tauri. 

Aldebaran. — An Arabic name for a fixed 
star of the first magnitude, situated in 
the constellation Taurus. It is popu- 
larly called the Bull's Eye. 

Alderamin.— A star in the constellation 
Cepheus, also known a Cephei. 

Aldhiba. — A star in the constellation Draco, 
and which is also called i Draconis. 

Algenib. — A star in the constellation Pe- 
gasus, known also as y Pegasi. The 
name of Algenib is also sometimes given 
to a Persei, a star in the constellation 
Perseus. 

Algieba. — A star in the constellation Leo, 
also known as y Leonis. 

Algol. — This star, known as /? Persei, is 
variable. Its period is rather less than 
three days. 

Algorab. — A bright star in the constella- 
tion Corvus, called also 6 Corvi. 

Alhaid. — See Benetnasch. 

Alidad or Alhidade. — An Arabic name for 
the index or ruler movable about the 
centre of an astrolabe. 

Alidade Level. — A spirit-level attached to 
the bar and small circle belonging to the 
instrument called the meridian circle. 

Alioth. — A star in the constellation Ursa 
Major, also called e UrsEe Majoris. 

Alkalurops. — A triple star in the constella- 
tion Bootes, commonly known as fx Bootis. 

Alkes. — A name given to a star in the con- 
stellation Crater, also called a Crateris. 

Almacantais, (from the Arabic Almocari- 
tharat.) — A name for the circles of alti- 
tude. These are circles parallel to the 
horizon, serving to show the height of 
the Sun, Moon, or stars. 

Almacantar StaiF. — An instrument having 
an arc of about 15°, used for observing 
the Sun or a star when near the horizon, 
to find the amplitude or the variation of 
the needle. 

Almagest. — A collection of problems in 



414 



BOUVIER's familiar ASTRONOxMY. 



geometry, and observations in astro- 
nomy, compiled by Ptolemy. 

Almak. — A star in the constellation An- 
dromeda, called y Andromedse. 

Almanac. — According to Golius, it is de- 
rived from the Arabic particle al, and 
manak, a reckoning. Scaliger derives it 
from the Arabic particle al, navaKos, the 
course of the months; but Vestigan as- 
cribes it to Saxon origin, believing it to 
be from the compound Saxon word Al- 
mon-ahr — that is, AU-moon-heed, or an 
account of every moon, which the Saxons 
are said to have kept very carefully. An 
almanac is a table or calendar, contain- 
ing an account of the months, weeks, &c. 
of the year, and also of the festivals. 

Almeisam. — The Arabic name for the star 
y Geminorum. 

Al-Mirzam. — See Gomeisa. 

Almucantar. — See Almacantar. 

Alnilham,. — A star of the second magni- 
tude in Orion's Belt, also known as e 
Orionis. 

Alnitak. — A star in the belt of Orion, gene- 
rally called ? Orionis. 

Alphard. — A name given to a star in the 
constellation Hydra, also known as a Hy- 
drse or Cor Hydrce. 

Alphecca or Gemma. — The brightest star 
in the constellation Corona Borealis, also 
known as a Coronas Borealis. 

Alpheratz or Sirrah. — A star in Andro- 
meda, also called a Andromedae. This 
star is the north-eastern boundary of the 
square of Pegasus. 

AlpMrk. — This star, called /? Cephei, is of 
the third magnitude in the constellation 
Cepheus. 

Alphonsine Tables. — See Tables. 

Alrucaba. — The same as Polar Star, which 
see. 

Alshain. — Another name for the star /? 
Aquilae, in the constellation Aquila. 

Altair. — A star of the first magnitude in the 
constellation Aquila, called also a Aquilae. 

Altimeter, {altus, high, and jitrpov, mea- 
sure.) — An instrument for measuring al- 
titudes ; as a quadrant, sextant, or theo- 
dolite. 

Altimetry. — The art of measuring alti- 
tudes. 



Altitude, (from altitudo, height.) — The 
height of an object above the horizon ; 
or, in other words, the arc of a vertical 
circle contained between the centre of 
the object and the horizon. 

Altitude and Azimuth Instrument. — An 
instrument used to find the place of a 
heavenly body in altitude and azimuth. 

Altitude, Apparent. — The distance of a 
heavenly body from the apparent or sen- 
sible horizon. 

Altitude Instrument. — The equal-altitude 
instrument is used to observe a celestial 
object when it has the same or an equal 
altitude on both sides of the meridian. 
It is useful in adjusting clocks. 

Altitude, Meridian. — See Meridian Alti- 
tude. 

Altitude of the Equator. — The angle or 
arc of the meridian intercepted between 
the horizon and the equator. It is equal 
to the co-latitude of the place. 

Altitude or Elevation of the Pole. — An 
arc of the meridian intercepted between 
the horizon and the pole of the heavens^ 
it is equal to the latitude of the place. 

Altitude of the Tropics.— Otherwise called 
the solstitial altitude of the Sun; his 
meridian altitude when in the solstitial 
points. 

Altitude, True. — The distance of a celes- 
tial body from the real or rational hori- 
zon. The correction of the apparent 
altitude on account of refraction and pa- 
rallax. 

Aludra. — A name applied to the star ;; 
Canis Majoris, in the constellation Canis 
Major. 

Alwaid. — A star in the constellation Draco, 
known also as Draconis. 

Alya. — Another name for a star in the con- 
stellation Serpens, known also aa 6 Ser- 
pentis. 

Amorphotae. — Unformed stars, or those 
stars which are not considered as in- 
cluded in any constellation. 

Amphiscii, (uj«c6«, on both sides, and aXia, a 
shadow.) — The inhabitants of the torrid 
zone are so called, because one-half the 
year their shadows point north, and the 
other half south. 

Amphitrite. — The twenty-ninth asteroid. 



ASTRONOMICAL DICTIONARY. 



415 



It was discovered by Marth, March 2, 
1854. 

Amplitude, {ampUtudo, extent.) — An arc of 
the horizon contained between the centre 
of the object when rising or setting and 
the east and west points of the horizon. 
It is the distance which a heavenly body 
rises from the east and sets from the west. 
Amplitude is ortive or eastern when the 
star is rising, and occiduous or western 
when it is setting. 

Analemma. — A narrow strip painted on 
the globe, the length of which is equal to 
the breadth of the torrid zone. It is di- 
divided into months, and days of the 
month, corresponding to the Sun's decli- 
nation for every day in the year. It is 
also divided into two parts; the right- 
hand one begins at the winter solstice, 
or 22d of December, and is reckoned to- 
wards the summer solstice, or the 21st of 
June, where the left-hand part begins, 
which is reckoned in a similar manner 
towards the winter solstice. By the ana- 
lemma the equation of time is shown. 

AnachroilisiXl, {dva, higher, and 'ppovog, time.) 
— An error in the computation of time, 
whereby an event is placed earlier than 
it really occurred. 

Ancha. — A name given to the star B Aqua- 
rii, in the constellation Aquarius. 

Andromeda. — A constellation in the north- 
ern hemisphere, so named by the Greeks 
in honor of Andromeda, the daughter of 
Cepheus. The right ascension and de- 
clination of the middle of the constella- 
tion are R. A. 0^. 56»!., Dec. 32° N. 

Angle, {angulusy a corner.) — The inclina- 
tion or opening of two lines, having dif- 
ferent directions, and meeting in a point. 

Angle, Acute, (acutus, sharp-pointed.) — 
An angle less than a right angle, or which 
does not subtend 90°. 

Angle of Commutation. — The angle at the 
Sun formed by two lines, the one drawn 
from the Earth, and the other from the 
place of a planet reduced to the ecliptic, 
both meeting in the Sun's centre. It is 
the diflference between the Sun's longi- 
tude and the heliocentric longitude of a 
planet. 

Angle of Contact. — See Contact. 



Angle of Elongation. — The angular dis- 
tance of any planet from the Sun with 
respect to the Earth; or the difference 
between the Sun's place and the geocen- 
tric place of a planet. 

Angle of Eccentricity. — The angle whose 
sine is equal to the eccentricity of an 
orbit. It is the angle formed at the ex- 
tremity of the minor axis of the ellipse 
by lines drawn to the centre and one of 
its foci, and is usually denoted by the 
Greek letter 0. 

Angle, Exterior. — In a polygon, the angle 
included between any side and the pro- 
longation of the adjacent one. In a tri- 
angle any exterior angle is equal to the 
sum of the two interior opposite ones. In 
any right-lined figure the sum of all the 
exterior angles is always equal to four 
right angles. 

Angle of Incidence. — That made by the 
line of direction of an impinging body at 
the point of impact. 

Angle of Longitude. — The angle which 
the circle of a star's longitude makes 
with the meridian at the pole of the 
ecliptic. 

Angle, Obtuse. — An angle greater than a 
right angle. It therefore contains more 
than 90°, and less than 180°. 

Angle of Position. — In a binary system 
of stars, it is the angle which the meri- 
dian, or a parallel to the equator, makes 
with the line joining the two stars. It 
is the angle formed by a line joining 
the two stars, with the meridian pass- 
ing through the larger one, and is reck- 
oned from 0° to 360°, beginning at the 
north point, and counting round by the 
east. 

Angle of Reflection.— That made by the 
line of direction of a reflected body at 
the point of impact. 

Angle, Right. — An angle formed by the 
meeting of two straight lines, one of 
which is perpendicular to the other. A 
right angle contains 90°, or the quarter 
of a circle. 

Angle of Right Ascension. — The angle 
which the circle of the star's right ascen- 
sion makes with the meridian at the pole 
of the equator. 



I 



416 



BOUVIER S FAMILIAR ASTRONOMY. 



Angle of Situation. — The angle made at 
a star by ares passing through the zenith 
and pole respectively. It is also termed 
the parallactic angle. 

Angle, Spherical. — An angle formed on the 
surface of a sphere by the intersection of 
two circles, or the inclination of the 
planes of those circles. 

Angle of the Vertical. — The diflforence be- 
tween the geographical and geocentric 
latitudes of a place. It is zero at the 
equator, and increases up to the 45th de- 
gree of latitude, when it amounts to 11' 
30", and thence diminishes to zero again 
at the pole. 

Angle of Vision. — The angle included be- 
tween two rays of light drawn to the eye 
from the extreme points of an object. 

Angles, Adjacent. — Those angles in the 
same plane which have one side in com- 
mon, and their other sides in the pro- 
longation of the same straight line. 

Angles, Contiguous. — Those angles which 
have the same vertex and a side common 
to both, but the other sides not in the 
same straight line. 

Angular Motion. — The motion of a body 
which moves circularly about a point; 
or the variation in the angle described 
by a line or radius connecting a body 
with the centre about which it moves. 

Angular Velocity. — The swiftness with 
which a body revolves ; or, the speed 
with which the surface of the Earth per- 
forms its daily rotation about its axis. 
As applied to a double star, it is the 
rate of motion of one star round the 
other. 

Annual. — Yearly, returning or ending with 
the year. 

Annual Argument of the Moon's Apogee. 
— See Argument. 

Annual Equation. — An inequality caused 
by a variation in the angular motion of 
the Moon, which becomes slower as the 
Earth and Moon are approaching the 
Sun, and accelerates as they recede from 
him. 

Annual Parallax.— The angle under which 
the diameter of the Earth's orbit would 
appear if viewed from a distant planet 
or a fixed star. 



Annual Revolution. — The yearly course 
of the Earth round the Sun, producing 
the phenomena of the seasons. 

Annual Variation. — The change produced 
in the right ascension or declination of a 
star by the precession of the equinoxes 
and proper motion of the star combined. 

Annular, (anmdus, a ring.) — Having the 
form of a ring. 

Annular Eclipse.— The eclipse of the 
whole orb of the Sun, except a ring or 
annulus, which appears round the border 
or edge. 

Anomalistic Revolution of the Moon. — 
The passage of the Moon from perigee 
to perigee, or from apogee to apogee. 

Anomalistic Year. — The interval of time 
between two consecutive returns of the 
Earth to her perihelion or aphelion ; or, 
the time in which the Earth or a planet 
passes through its orbit with regard to 
its line of apsides. 

Anomaly, {dv6na\ia, irregularity.) — The an- 
gular distance of a planet from aphelion 
or apogee; that is, the angle formed by 
the line of the apses, and another line 
drawn through the planet. 

Anomaly, Eccentric. — An auxiliary angle 
employed to abridge the calculations con- 
nected with the motion of a planet or 
comet in an elliptic orbit. If a circle be 
drawn, having its centre coincident with 
that of the ellipse, and a diameter equal 
to the transverse axis of the latter, and 
if from this axis a perpendicular be drawn 
through the true place of the body in the 
ellipse to meet the circumference of the 
circle, then the eccentric anomaly will be 
the angle formed by a line drawn from 
the point where the perpendicular meets 
the circle, to the centre, with the longer 
diameter of the ellipse. — Hind. 

Anomaly, Mean. — The angular distance 
of a planet or comet from perihelion or 
aphelion, supposing it to have moved 
with its mean velocity. 

Anomaly, True or Equated. — The angular 
distance of a planet or comet from the 
perihelion or aphelion. 

Ansae or Anses, {ansce, handles.) — The ap- 
parently prominent parts of the ring of 
the planet Saturn. 



ASTRONOMICAL DICTIONAEY. 



417 



Anser Americanus. — Another name for 
Toucana, which see. 

Antarctic Circle. — An imaginary, small 
circle, parallel to the equator, and 23° 
28' from the South Pole. 

Antartic Pole, {avn, against or opposite to, 
and apKTo^, a hear.) — The southern extre- 
mity of the Earth's axis. 

Antares. — The principal star in the con- 
stellation Scorpio, sometimes called Cor 
Scorpionis, Calbalacrab, and a Scorpii. 

Antecedentia. — A term signifying a retro- 
grade motion of a planet; that is, con- 
trary to the order of the signs. 

Antemeridian, {ante, before, and meridies, 
noon.) — The time before noon. Abbre- 
viated thus, A. M. 

Anthelion, (avn, opposite, and ri\ios, the 
Sun.) — In the northern portions of our 
globe the Sun and Moon frequently ap- 
pear surrounded by halos or colored cir- 
cles. "When a horizontal white circle 
intersects these halos, bright spots, re- 
sembling the Sun, appear near their in- 
tersection. In the horizontal circle there 
are often other bright spots, nearly oppo- 
site to the Sun; these are called anthelia. 

Antimeter. — An instrument used for the 
accurate measurement of angles. 

Antinous. — A northern constellation, gene- 
rally reckoned as part of the constella- 
tion Aquila. Its R. A. is 19h. 28m., and 
its Dec. 0° 0'. 

Antipodes, {avn, opposite to, and noSog, the 
foot.) — The inhabitants of two places on 
the Earth which lie immediately oppo- 
site to each other, or where they walk feet 
to feet. 

Antiscians, (««"■', opposite to, and ax^a, a 
shadoio.) — Those who, living in opposite 
hemispheres, have their shadows at noon 
directly opposite to each other. 

Antlia Pneumatica.— One of the southern 
constellations, introduced by Lacaille. 
Its R. A. is 10^. Om., and Dec. 35° 0' S. 

Antoeci, (a^'■l, opposite to, and oixos, a 
house.) — Those inhabitants of the Earth 
who live under the same meridian east 
or west, but under opposite parallels of 
latitude north or south. 

Antolycus. — A crater or cavity in the lunar 
surface, seen only through the telescope. 



Apastron, {dno, from, and agrpov, a star.) — 
That point in the orbit of a double star 
which is farthest from its primary. 

Aperture. — That portion of the object-glass 
of a telescope through which objects may 
be viewed, which is generally rather less 
than the tube of the instrument. Also, 
the opening of the tube or cylinder in 
which the object-glass is inserted. 

Apex. — The summit or highest point of any 
thing ; the vertex. The apex of a cone 
is the highest point above the base. 

Aphelion, {d-jd,from., and rihog, the Sun.) — 
That point in the orbit of a planet or 
comet which is most distant from the 
Sun. 

Aplanatic, (a, deprived of, and i:\auri, error.) 
— Void of error. A term applied to those 
optical instruments in which the sphe- 
rical and chromatic aberrations are cor- 
rected. 

Apogee, (drrd, from, and yri, the Earth.) — 
That point of the Moon's orbit farthest 
from the Earth. 

Apogean Tides. — Those neap tides which 
occur soon after the Moon passes her 
apogee. 

Apparent, {appareo, to appear.) — A term 
applied to things as they appear to the 
eye, in distinction from what they really 
are. 

Apparent Conjunction. — The situation of 
two heavenly bodies being such that a 
right line, supposed to be drawn through 
their centres, would pass through the eye 
of a spectator, and not through the centre 
of the Earth. 

Apparent Diameter or Magnitude. — The 
angle which the heavenly bodies subtend 
at the eye. 

Apparent Distance. — The distance which 
we judge an object to be when seen from 
afar ; thus all the heavenly bodies appear 
at the same distance from us, although 
they are situated many millions of miles 
asunder. 

Apparent Equinox. — The position of the 
equinox as affected by nutation. 

Apparent Horizon. — The circle which 
bounds our view on all sides ; the sen- 
sible horizon. 

Apparent Motion. — The motion of the 



27 



418 



BOUYIER S FAMILIAR ASTRONOMY. 



heavenly bodies as seen from our globe ,: 
or that motion which we see in a distant 
body which is moving or at rest, while 
the eye is either at rest or in motion. 

Apparent Noon. — The time when the Sun 
comes to the meridian — viz., 12 o'clock, 
as shown by a correct sundial. 

Apparent Obliquity. — The inclination of 
the equinoctial to the ecliptic, calculated 
with nutation. 

Apparent Place. — That point where the 
centre of a heavenly body appears when 
viewed from the surface of the Earth. 

Apparent Sidereal Day.— See Sidereal 
Day. 

Apparent Sidereal Time. — Time measured 
by the hour-angle of the apparent equi- 
nox. 

Apparent Solar Day. — The time elapsed 
from 12 o'clock at noon on any day till 
12 o'clock at noon on the nest day, as 
shown by a correct sundial. 

Apparent Time. — The measure of any du- 
ration rendered sensible by means of 
motion. 

Apparent Zenith.. — See Zexith. 

Apennines. — A chain of lunar peaks visi- 
ble through the telescope. 

Appulse, {appello, to drive to.) — The near 
approach of two heavenly bodies so as 
to be seen at the same time in the field 
of the telescope. 

April. — The fourth month in the year, and 
the second from the vernal equinox. 
This word is derived from aperio, to 
open; because the Earth in this month 
opens her bosom for the production of 
plants. 

Apsides or Apses, {i'-^k, a connection.) — 
The extremities of the major axis of an 
orbit ; that is, the point which a planet 
occupies at its greatest and least dis- 
tances from the Sun ; also those points 
at which a satellite is at its greatest and 
least distances from its primary. See 
LiXE OF Apsides. 

Apsis, Higher. — That point in the orbit of 
the Earth or of a planet which is farthest 
from the Sun ,: its aphelion. 

Apsis. Lower. — That point in the orbit of 
the Earth or of a planet which is nearest 
to the Sun; its perihelion. 



Apus vel Avis Indica. — A southern con- 
stellation, the middle of which is R. A. 
16^. 44;»., Dec. 73° 0' S. 
Aquarius. — The eleventh sign in the zo- 
diac, and is marked thus 7^. R. A. 22h. 
20m., Dec. 11° 0' S. 
Aquila. — A northern constellation. R. A. 

19A. 40;n., Dec. 10° C N. 
Ara. — A constellation in the southern hemi- 
sphere, a little to the south-east of Scor- 
pio. R. A. 17A. 0?rt., Dec. 55° 0' S. 
Aramech. — Another name for Arcturus. 
Arc, {arcus, a bow.) — A part of any curved 
line, as of a circle, ellipse, <tc, 
, Arc of Direction or Progression. — That 
I arc which a planet appears to describe 
{ when its motion is direct or progressive. 
i Arc, Diurnal. — That part of a circle pa- 
rallel to the equator, described by the 
Sun from his rising to his setting. 
Arc, Nocturnal. — That part of a circle 
parallel to the equator, described by the 
Sun or a star from setting to rising. 
Arc of Retrogradation. — The arc of the 
ecliptic described while a planet is retro- 
grade, or moves contrary to the signs ; 
that is, from east to west. 
Archimedes. — A spot on the surface of the 
Moon visible by the aid of the telescope. 
Arctic Circle, (apxroj-, a bear; because the 
constellation of the Great Bear is in 
its vicinity.) — One of the lesser circles 
'< of the globe, at the distance of 23° 2S' 
from the North Pole. It is also called 
the North Polar Circle. 
Arctic Pole. — The northern extremity of 

the Earth's axis. 
Arctophylax, (bear-keeper.) — A name 
j given by the ancients to the constella- 
I tion Bootes. 

■ Arcturus. — A star of the first magnitude 
i in the constellation Bootes. R. A. lih. 
Sm., Dec. 20° N. 
Area. — Surface, superficial extent 
Argo Navis. — A southern constellation. 
; R. A. 7h. Urn., Dec. 50° S. As this con- 
I stellation is unusually large, Sir John 
Herschel divided it into four parts — viz., 
Carnia, the keel; Puppi^, the stem; Vela, 
the sails: and Argo Xavis, the hulL 
Some make another division — Malus, the 
mast. 



ASTRONOMICAL DICTIONARY. 



419 



ATgument. — An arc given, by which an- 
other arc is found proportional to it. 

Argument, Annual of the Moon's Apo- 
gee. — The distance of the Sun's place 
from the Moon's apogee ; that is, an arc 
of the ecliptic comprised between these 
two places. 

Argument of Latitude or Argument of 
Inclination. — The arc of a planet's orbit 
intercepted between the ascending node 
and the place of the planet from the Sun, 
numbered according to the succession of 
the signs. 

Argfument of Latitude, Menstrual.— The 
distance of the Moon's true place from 
the Sun's true place. 

Arided. — A bright star in the constellation 
Cygnus, also called a Cygni. 

Ariel. — A name given by Sir John Herschel 
to the first satellite of Uranus. 

Aries. — The first of the twelve signs of the 
zodiac, and is marked thus T, in imita- 
tion of a ram's head. The first point of 
Aries is the origin from which the right 
ascensions of the heavenly bodies are 
reckoned upon the equator, and their 
longitudes upon the ecliptic. 

Arietis. — The chief star in the constella- 
tion Aries ; it is also called a Arietis, and 
by the Arabs was called Hamal. 

Aristarchus. — A bright spot or prominent 
point on the surface of the Moon, as seen 
through a telescope. 

Aristillus. — One of the lunar craters, visi- 
ble by means of a telescope. 

Aristotle. — A lunar inequality made visi- 
ble by the telescope. 

Armillary Sphere, {armilla, a bracelet.) — 
It is an instrument composed of a num- 
ber of circles of metal, wood, or paper, 
representing the several circles of the 
sphei-e, and which assists in the concep- 
tion of the disposition and motions of the 
heavenly bodies. 

Ameb. — A name applied to a star in the con- 
stellation Lepus, also known as a Leporis. 

Arrakis. — Another name for a star in the 
constellation Draco ; it is also called n 
Draconis. 

Ascending. — A term used to indicate the 
rising of a heavenly body or point of the 
heavens above the horizon. 



Ascending Constellations. — Those con- 
stellations through which the planets 
ascend northwardly. 

Ascending Latitude. — The latitude of a 
planet when going north. 

Ascending Node. — That point in a planet's 
orbit where it intersects the ecliptic pro- 
ceeding northward. It is marked thus Q>. 

Ascending Signs. — Those signs of the zo- 
diac from the Tropic of Capricorn to that 
of Cancer are called the ascending signs. 
Also, those signs which are eastward of 
the meridian, and by the diurnal rota- 
tion of the Earth are approaching the 
zenith. 

Ascension. — See Right and Oblique. 

Ascension of the Midheaven.— The right 
ascension of that point of the equinoctial 
which is on the meridian. 

Ascensional Difference. — The difference 
between right and oblique ascension of 
the same point on the surface of the 
sphere. It is the time the Sun rises or 
sets before or after six o'clock. 

Ascii, (from aaKiog, icitliout shadow.) — This 
name is applied to the inhabitants of the 
torrid zone, who twice a year have the 
Sun in their zenith at noon, and then 
have no shadow. 

Asellus Australis.— A star in the constel- 
lation Cancer, also called 6 Cancri. 

Asellus Borealis. — A star in the constella- 
tion Cancer, known as y Cancri. 

Ashtaroth. — A name given by the ancients 
to the star a Coronas Borealis. 

Asmidiske. — A name given to the star I, 
in the constellation Argo Navis. 

Aspect. — The situation of the stars or pla- 
nets with regard to each other. They 
are as follows : 



Name. 


Character. 


Distance. 


Conjunction, 


6 


(P 


Sextile, 


* 


600 


Quartile, 


n 


90° 


Trine, 


A 


120° 



8 



180° 



Opposition, 

Aspidiske. — A name given by Ptolemy to 

the star i Argo Navis. 
Asterion et Chara.— See Canes Vexatici. 
Asterism. — A constellation; a group or 

collection of stars. 
Asteroids or Planetoids. — This name was 



420 



BOUVIER S FAMILIAR ASTRONOMY. 



given by Herschel to the small planets 
which revolve between the orbits of Mars 
and Jupiter. They are sometimes called 
the ultra-zodiacal planets. 

Asterope. — A name given to one of the 
Pleiades. 

Astral. — Relating to the stars. 

Astrea. — The fifth asteroid,- it was dis- 
covered by Hencke, December 8, 3845. 

Astrognosy. — The science of the stars, or 
knowledge of their magnitudes, situa- 
tions, &c. 

Astrography, (ar/voi/, a star, and ypacpw, to 
write.) — A description of the stars. 

Astrolabe, {aTpov, a star, and Xa/Seiv, to take.) 
— A system or assemblage of the several 
circles of the sphere in their proper order 
and situation with respect to each other. 
It was first made by Hipparchus, at Alex- 
andria, in Egypt, and was afterwards 
modified by Ptolemy; so that the whole 
was reduced to a plane surface, which he 
called a planisphere. It was formerly 
used for taking observations at sea. 

Astrolithology, {arpov, a star, and \iQo^, a 
stone.) — That branch of science pertain- 
ing to meteoric stones or meteorites. 

Astrometer, {a~pov, a star, and utrpov, a mea- 
sure.) — An instrument used to compare 
the intensities of the light of different 
stars; one who measures the light of the 
stars. 

Astrometry. — The numerical expression 
of the apparent magnitudes of the fixed 
stars. 

Astronomical Characters. — Those charac- 
ters used in astronomy. See Chaeac- 

TERS. 

Astronomical Horizon. — The same as Ra- 
tional Horizon, which see. - 

Astronomical Hours. — Those hours which 
are reckoned from noon, or midday of 
one day, to noon of the following day, in 
a continued series of twenty-four. 

Astronomical or Natural Day. — The time 
which elapses from noon to noon, which 
consists of twenty-four hours. 

Astronomical Observations. — Those obser- 
vations of the heavenly bodies made by 
astronomers in order to ascertain their 
forms, appearances, motions, <fec. 

Astronomical Place. — The longitude or 



place of a body in the ecliptic, reckoned 
from the first point of Aries, according 
to the order of the signs. 

Astronomical Tables. — See Tables. 

Astronomical Telescope. — A telescope used 
only in astronomical observations. 

Astronomy, [aTpov, a star, and vonog, a law.) 
The science which treats of the heavenly 
bodies and the laws by which they are 
governed. 

Astroscope, (aTpov, a star, and CKonew, to 
view.) — A kind of astronomical instru- 
ment composed of telescopes, invented in 
1698 by William Shukhard, Professor 
of Mathematics at Tubingen. Also, an 
instrument composed of two cones, hav- 
ing the constellations delineated on theii 
surfaces : not now used. 

Asymptote, (a, deprived of, and avyminTOi, 
to meet.) — A name for lines which con- 
tinually approach each other, but which, 
if infinitely produced, can never meet. 

Atik. — A star in the constellation Perseus, 
called also o Pegasi. 

Atlantides. — Another name for the Plei- 
ades. 

Atmosphere, {arixog, vapor, and a^aXpa, 
sphere, or region.) — That invisible elastic 
fluid which surrounds the Earth to an un- 
known height, and encloses it on all sides. 

Atom, (a, deprived of, and t£[xvco, to divide.) 
— An indivisible particle of matter. 

Atomic Theory. — That doctrine of philo- 
sophy which explains the origin of all 
things by a combination of atoms. It 
was first taught by Mochus, of Sidon, 
who lived before the Trojan war: others 
are of opinion it was established by Leu- 
cippus, 510 B. c. Newton and Boerhaave 
supposed that the original matter con- 
sists of hard, ponderable, impenetrable, 
and inactive particles, the variety and 
composition of which causes the differ- 
ence in bodies. A system founded on 
the theory of atoms is called atomic, and 
sometimes corpuscular, philosophy. 

Attraction, {attraho, to draw to.)— That 
power or principle in matter by which all 
bodies mutually tend towards each other. 

Attraction of Gravitation. — That force by 
which bodies descend towards the centre 
of the Earth. 



ASTRONOMICAL DICTIONARY. 



421 



Augmentation of Moon's Semi-diameter. 

— The increase due to the diflference be- 
tween her distance from the observer and 
the centre of the Earth. 

August — The eighth month of the year, 
called after the Emperor Augustus, who 
entered his second consulship in that 
month, after the Actian victory. Before 
that time it was called Sextilis. Our 
Saxon ancestors called it weod-monath, 
weed month. 

Auriga. — One of the old northern constel- 
lations. R. A. 5h. Om., Dec. 45° 0' N. 

Aurora. — The morning twilight. 

Aurora BoreaUs, Aurora Septentrionalis, 
or Northern Lights.— A kind of meteor, 
appearing in the northern part of the 
heavens mostly in the winter season. It 
is generally believed to be an electrical 
phenomenon, although its origin is not 
certainly known. 

Austral. — Southern. The six signs of the 
zodiac south of the equinoctial are called 
the Austral signs. 

Autumn. — The third season of the year in 
the northern hemisphere, which com- 
mences at the descending equinox, when 
the Sun enters Libra. 

Autumnal or Descending Equinox. — The 
time when the Sun enters Libra. 

Axis, Conjugate. — That axis of an ellipse 
which bisects the transverse axis. It is 
the shorter of the two axes. 

Axis, Declination. — The declination axis 
of a telescope is that axis of motion 
which is parallel to the plane of the 
equator. 

Axis of a Lens. — An imaginary straight 
line, drawn from the centre of a sphe- 
rical surface to the centre of that circle 
of which its surface forms an arc. 

Axis, Major. — See Axis of an Orbit. 

Axis, Minor. — See Axis of an Orrit. 

Axis of an Orbit. — The line joining its 
perihelion and aphelion points ; this is 
sometimes called the major axis. A 
line perpendicular to this, and passing 
through the centre of the ellipse, is called 
the minor axis. 

Axis of a Planet, (afwi^, axis.) — An imagi- 
nary line passing through its poles, upon 
which it revolves. 



Axis, Polar. — That axis of motion of a 
telescope which is parallel to the axis 
of the Earth. 

Axis of a Telescope. — An imaginary line 
passing through the centre of the tube. 

Azelfafage. — Another name for the star rr, 
in the constellation Cygnus. 

Azha. — Another name for »? Eridani, a star 
in the constellation Eridanus. 

Azimuth. — The arc of the horizon con- 
tained between a vertical circle passing 
through the object, and the north or 
south points of the horizon. The word 
azimuth is pure Arabic, and signifies to 
move or go forward. 

Azimuth Circles. — Great circles of the 
Sphere passing through the zenith and 
intersecting the horizon at right angles. 

Azimuth Compass. — An instrument for 
finding either the magnetical azimuth or 
amplitude of a circle at sea. A compass 
used at sea for finding the horizontal dis- 
tance of a celestial body from the mag- 
netic meridian. 

Azimuth, Magnetic. — An arc of the hori- 
zon contained between the magnetic 
meridian and the azimuth or vertical 
circle of the object. 

Azimuthal Error, sometimes termed the 
Meridian Error. — The deviation of a 
transit instrument from the plane of the 
meridian. Its maximum is at the hori- 
zon, and at the zenith it is zero. 

B. — In the astronomical tables B stands 
for Bissextile or Leap year. 

Back StaflF. — An instrument formerly used 
for taking the Sun's altitude at sea ; so 
called because the back of the observer 
is turned towards the Sun during the ob- 
servation. It was invented about the 
year 1590, by John Davis, a Welshman. 

Baculometry, (baculus, a staff, and iitrpov, 
a measure.) — The art of measuring alti- 
tudes by means of a staff. 

Balance. — Another name for the sign Libra. 

Barometer, {Papv^, heavy, and nirpov, a mea- 
sure.) — An instrument invented by Tor- 
ricelli for measuring the weight or pres- 
sure of the atmosphere ; and, b}' means 
of the variations in the air, of foretelling 
changes in the weather. It is also used 



422 



BOUVIER S FAxMILIAR ASTRONOMY. 



for ascertaining the heights of moun- 
tains. 

Base. — In geometry, the lowest side of the 
perimeter of a figure. Of a solid figure, 
the side on which it stands. 

Base Line. — In surveying, a line measured 
with great exactness, on which a series 
of triangles is constructed in order to de- 
termine the distances and positions of 
objects. Astronomers make use of the 
Earth's diameter, and sometimes of the 
diameter of the Earth's orbit, for a base 
line. 

Basilica or Basiliskos. — ^Another name for 
Begulus, which see. 

Baten Kaitos. — Another name for ^ Ceti, a 
star in the constellation Cetus. 

Bear, Great. — See Ursa Major. 

Bear, Lesser.— See Ursa Minor. 

Beard of a Comet. — The rays it emits in 
the direction in which it moves, as dis- 
tinguished from the tail, or the rays 
emitted or left behind as it moves a^png. 

Beid. — A name given to o Eridani, a star 
in the constellation Eridanus. 

Bellatrix. — A bright star in the left shoul- 
der of Orion. Its name is from the Latin 
helium, as being supposed by the ancients 
to have an influence over warriors, and 
to kindle wars. It is also called y Orionis. 

Bellona. — The twenty-eighth asteroid. It 
was discovered by Luther, March 1, 1854. 

Belts. — The dark bands which the tele- 
scope reveals on the bodies of the planets 
Jupiter and Saturn. 

Benetnasch or Alhaid. — A bright star of 
the second magnitude in the extremity 
of the tail of Ursa Major, also called rj 
Ursse Majoris. 

Bengalee Year. — This was once almost 
identical with the Hegira; but since the 
adoption of the solar computation it is 
now about nine years later than the He- 
gira. To bring the Bengalee year to the 
Christian era, the number 593 must be 
added. 

Berenice's Hair. — See Coma Berenices. 

Betelgeux. — A fixed star of the first mag- 
nitude in the right shoulder of Orion, also 
termed a Orionis. 

Biangular, {bis, twice, and angidm, an 
angle.) — Having two angles. 



Bifid. — A term applied to the tails of comets 
which are separated into two branches. 

Binary System. — A system of two stars re- 
volving about a common centre of gravity. 

Binocular Telescope, {bini, two together, 
and ocidiis, an eye.) — A telescope through 
which an object may be viewed by both 
eyes at the same time; now not used. 

Binuclear. — A nebula having two nuclei 
is said to be binuclear. 

Biquintile. — An aspect of the planets when 
they are 144° from each other. 

Bisect, {bis, twice, and seco, to cut.) — To 
divide any thing into two equal parts. 

Boreal Signs. — The first six signs of the 
zodiac, or those on the northern side of 
the equinoctial — viz., Aries, Taurus, Ge- 
mini, Cancer, Leo, and Virgo. 

Bissextile or Leap Year. — A year consist- 
ing of S66 days. It occurs every fourth 
year, when a day is added to the month 
of February, which in that year consists 
of 29 days. 

Bootes. — One of the old northern constel- 
lations, the middle of which is R. A. 147». 
2Sm., Dec. 30° 0' K 

Borealis. — Northern. See Aurora Bo- 

REALIS. • 

Botein. — Another name for s Arietis, in 

the constellation Aries. 
Brilliant Point. — That point from which 

the light of a surface is reflected to the 

eye in a direct line. 
Brush. — A term applied to a comet's tail ; 

and more especially when the tail is 

divided. 
Bulk. — Volume : the relative bulk of two 

globes of unequal magnitude are to each 

other as the cubes of their diameters. 
Bull's Eye. — By the Arabs caUed Alde- 

baran, which see. 
Bungula. — A bright star in the constella- 
tion Centaurus, and marked a in the 

catalogues and maps. 

Calhalacrab or Kalb-al-akrab. — Another 
name for Antares, a bright star in the 
constellation Scorpio. 

Calendar. — Among the Romans the appear- 
ance of new moon was watched and pub- 
lie notice given of it. Hence the first 
day of every month was called calends. 



ASTRONOMICAL DICTIONARY. 



423 



It is now understood to be a distribution 
of time as accommodated to the uses of 
life; an almanac containing ttie order 
of the days, weeks, <fcc., in the year. See 
Julian Year and Gregorian Year. 

Calippic Period. — A period of seventy-six 
years, invented by Calippus, an Athenian 
astronomer, as an improvement on the 
Metonic cycle of nineteen years. At 
every recurrence, of this period he sup- 
posed that the mean new, and full moons 
would always return to the same day 
and hour. 

Calliope. — The twenty-second asteroid. It 
was discovered by Hind, November 16, 
1852. 

Camelopardalus. — A northern constella- 
tion, the middle of which is R. A. 4A. 
30»i., Dec. 70° 0' N. 

Cancer. — One of the twelve signs of the 
zodiac, the middle of which is R. A. 8h, 
24»i., Dec. 20° 0' N. 

Canes Venatici. — A northern constellation 
in R. A. 13;?, 12m., Dec. 40° 0' N. It is 
sometimes called Asterion et Chara. 

Canicula. — A name given by the earlier 
astronomers to the constellations now 
called Canis Major and Canis Minor; 
also to Sirius, which is sometimes called 
the dog star. 

Canicular Bays, [Dies Caniculares or Dog 
Days.) — A certain number of days be- 
fore and after the heliacal rising of cani- 
cula, or the dog star, in the morning. 

Canicular Year. — The Egyptian natural 
year, which was computed from one he- 
liacal rising of Sirius, or the dog star, to 
the next. 

Canis Major. — A southern constellation. 
R. A. 6/1. 20m., Dec. 22° 0' S. 

Canis Minor. — A northern constellation. 
R. A. n. 22m., Dec. 5° 30' N. 

Canopus. — A brilliant star of the first mag- 
nitude in the constellation Argo Navis, 
also known as a Argo Navis. 

Capella. — A star of the first magnitude in 
the constellation Auriga, known as a 
Aurigae. It was called by the Arabs 
Hirciis or Capra, which means the goat. 

Caph. — A star of the third magnitude in 
the constellation Cassiopeia, remarkable 
as being situated on the equinoctial co- 



lure. This star is also called /i Cassio- 
peiae. 

Capra. — See Capella. 

Capricornus. — One of the signs of the zo- 
diac, the middle of which is R. A. 20^. 
40m., Dec. 20° 0' S. 

Caput Medusae. — A part of the constella- 
tion Perseus. 

Cardinal Points of the Ecliptic. — The 
equinoctial and solstitial points — viz., 
Aries and Libra, Cancer and Capricorn. 

Cardinal Points of the Heavens. — The 
zenith, the nadir, and the points where 
the Sun rises and sets. 

Cardinal Points of the Horizon. — The 
east, west,^orth, and south points of the 
horizon. 

Cardinal Signs. — Those signs at the equi- 
noxes and solstices — viz., Aries, Libra, 
Cancer, and Capricorn. 

Carina. — A name given to the keel of the 
ship in the constellation Argo. 

Cassiopeia. — A northern constellation, the 
middle of which i.s R. A. Oh. 40?h., Dec. 
60° 0' N. 

Castor. — A double star of the second mag- 
nitude in the constellation Gemini, also 
called a Geminorum. 

Castor and Pollux. See Gemini. 

Catalogue. — A list of the principal fixed 
stars, in which may be found their right 
ascensions and declinations, as well as 
their latitudes and longitudes, with their 
variations, magnitudes, and proper mo- 
tions. 

Catoptrics. — That branch of optics which 
treats of the progress of rays of light 
after they are reflected from surfaces 
either plane or curved, and the forma- 
tion of images from objects placed before 
such surfaces. 

Cauda Leonis. — A fixed star of the first 
magnitude in the constellation Leo. See 
Denebola. 

Cela Sculptoria. — A small southern con- 
stellation. R. A. U. 44m., Dec. 40° 0' S. 

Celaeno. — A name given to one of the Plei- 
ades. 

Celestial Globe. — A representation of the 
heavens on an artificial globe, the prin- 
cipal stars being mapped out. 

Celestial Latitude, Circle of. — A great cir- 



424 



BOUVIER S FAMILIAR ASTRONOMY. 



cle of the sphere passing through the 
poles of the ecliptic, and consequently 
perpendicular to it. 

Celestial Latitude, Parallels of.— Small 
circles of the celestial sphere parallel to 
the ecliptic. 

Centaums. — A southern constellation situ- 
ated R. A. 137i. 30«j., Dec. 50° 0' S. 

Centenary. — Belonging to a century, or a 
period of a hundred years. 

Centesimal Division. — A division by hun- 
dreds. In astronomical tables the cir- 
cumference of the circle is sometimes 
divided into 400 degrees, each degree 
into 100 minutes, and ea'ch minute into 
100 seconds. 

Central, {centralis, placed in the centre.) — 
Pertaining to the centre. 

Central Eclipse. — This occurs when a line 
joining the centres of the Sun and Moon, 
and extended, would reach the eye of 
the spectator. 

Central Forces. — Those forces which cause 
a moving body to tend towards or recede 
from the centre of motion ; which are 
distinguished into two kinds — namely, 
centrifugal and centrijietal force. See 
Cextrifugal and Centripetal. 

Central Latitude.— See Latitude. 

Central Zenith. — See Zenith. 

Centre. — That point in a circle from which 
all lines drawn to the circuniference are 
equal. 

Centre of Gravity. — That point about 
which all the parts of a body do in any 
situation exactly balance each other. 

Centre of Motion. — That point which re- 
mains at rest while all the points of a 
body move about it. The centre of motion 
of a ship is the point upon which, when 
in motion, the vessel oscillates or rolls. 

Centrifugal, {centrum, a centre, and fugio, 
to fly from.) — That power which drives 
a revolving body from the centre. 

Centripetal, {centrum, a centre, and peto, 
to seek.) — That power which attracts a 
revolving body to the centre of motion. 

Century, {centum, a hundred.) — A hundred 
years. 

Cepheus. — One of the northern constella- 
tions, the middle of which is R. A. 22h. 
Om., Dec. 70° 0' N. 



Cerberus.— A northern constellation, the 
middle of which is R. A. ISA. Oni., Dec. 
22° 0' K 

Ceres. — The first asteroid. It w^^ disco- 
vered by Piazzi, January 1, 1801. 

Cetus. — A southern constellation, the mid- 
dle of which is R. A. Ih. 40?n., Dec. 12° S. 

Chameleon. — A southern constellation. R. 
A. llh. 0/H., Dec. 78° S. 

Characters. — Certain marks used by astro- 
nomers to denote the planets, signs of 
the zodiac, aspects, &c. O Sun; § Mer- 
cury ; ? Venus; © Earth; S Mara; 
(T)(T), &c., the first, second, &c. asteroid; 
% Jupiter; h Saturn; ^ Uranus; 
Neptune; T Aries; B Taurus; II Ge- 
mini; O Cancer; Q, Leo ; W Virgo; ^^ 
Libra; 1T\ Scorpio; ^ Sagittarius; l> 
Cajpricornus ; ^ Aquarius; X Pisces; 
6 Conjunction ; 8 Opposition ; A Trine ; 
n Quartile; ^ Sextile ; G New Moon; 
O Full Moon; J) Moon's first quarter ; <L 
^loon's last quarter ; fi Ascending node ; 
X^ Descending node ; '^ Star ; ® Globular 
cluster; O Planetary nebula; C 3Ioon 
above the horizon; d C Moon very bright; 
I., II., III., IV., Class one, two, three, 
four, of Sir W. Herschel's catalogue of 
nebtdce and clusters; 10, 25, 38, &c., 
number ten, twenty-five, thirty-eight, &c. 
of a class in the same catalogue. Thus, 
II. 45 means class two, number forty -five, 
of Sir W. Herschel's catalogue. A sig- 
nifies " Difi"erence," or " North Polar Dis- 
tance." A a means diflference of right as- 
cension, and A (5 diflference of declination. 
* A signifies " Star's Polar Distance." 
Jf is used to denote a comet. Thus, J^ o 
or Jf- (5 signifies the comet's right ascen- 
sion or declination. ° degree, ' minute, 
and " second, of angular measurement. 

Cheleb. — Another name given to the star 
/? Ophinchi, in the constellation Ophiu- 
chus. 

Chord, {x°P^^y and the Latin word chorda, 
a string of a bow or musical instrument.) 
— A right line connecting the two ex- 
tremes of an arc ; so called from its re- 
semblance to the chord or string of a bow. 

Chronology, {Xfiovo^, time, and ^oyog, law or 
science.) — The science which teaches the 
measure and division of time. 



ASTRONOMICAL DICTIONARY. 



425 



Chronometer, {xp°^os, time, and fierpov, a 
measure.) — An instrument used for mea- 
suring time, as a clock, watch, &c. But 
the term is more particularly applied to 
a kind of clock so constructed as to mea- 
sure minute divisions of a second. 

Circe. — The thirty-fourth asteroid ,• disco- 
vered by Chacornac, April 6, 1855. 

Circinus. — A small southern constellation. 
R. A. Ibh. Om., Dec. 60° 0' S. 

Circle. — A plane figure bounded by a curved 
line called the circumference, which is 
everywhere equidistant from a point 
within it called the centime. 

Circle, Almacantar, or Circle of Altitude. 
— A lesser circle of the sphere drawn 
parallel to the horizon. 

Circles of Altitude. — Small circles parallel 
to the horizon, which serve to show the 
height of the Sun, Moon, or stars, above 
the horizon. See Almacantar. 

Circle of Celestial Longitude.— The eclip- 
tic is called the circle of celestial longitude. 

Circles, Concentric— Those circles which 
have the same centre. 

Circles of Declination. — Great circles of 
the sphere intersecting each other at the 
poles, and perpendicular to the equinoc- 
tial, on which declination is measured. 
This term denotes a small circle parallel 
to the equator. 

Circle of the Disc. — That circle which di- 
vides the hemisphere of the Moon which 
is turned towards us from the hemisphere 
which is turned from us. 

Circle, Great. — That which divides the 
sphere into two equal hemispheres, hav- 
ing the same centre and diameter with it. 

Circle, Hour. — A small brazen circle fixed 
to the North Pole on the artificial globe, 
and divided into twenty-four parts or 
hours. It is furnished with an index, 
•which points out the diflference of meri- 
dians in time, and serves for the solution 
of many problems. In astronomy, the 
hour circle is a meridian of the Earth 
extended to the heavens, and passing 
through every 15° of the equinoctial. 

Circle of Illumination. — That imaginary 
circle on the surface of the Earth which 
separates the illuminated from the dark- 
ened hemisphere. 



Circle of Celestial Latitude.— See Celes- 
tial Latitude. 

Circle, Lesser. — A circle which divides a 
sphere into two unequal parts, having 
neither the same centre nor diameter with 
the sphere. 

Circle of Perpetual Apparition. — A lesser 
circle parallel to the equator, and de- 
scribed by the most northern point of 
the horizon. All the stars included 
within this circle are continually above 
the horizon of any given place, and there- 
fore never set. 

Circle of Perpetual Occultation. — A small 
circle parallel to the equator, and de- 
scribed by the most southern point of 
the horizon. It contains all the stars 
which never appear or rise in the hemi- 
sphere of any given place. 

Circles, Polar. — Two small circles parallel 
to the equator, at the distance of 23° 28' 
from the poles. The northern is called 
the Arctic, the southern the Antarctic, 
circle. 

Circle of Position. — A circle drawn through 
the intersections of the horizon and the 
meridian, and through a star, or any point 
of the sphere. 

Circle, Primitive. — The great circle cut 
from the sphere by the plane on which 
the projection is made. 

Circle of Right Ascension. — The same as 
the equator. 

Circles of the Sphere. — Circles which cut 
or pass through the sphere and have their 
circumference upon its surface. 

Circular Velocity. — The motion of a re- 
volving body, measured by an arc of a 
circle. 

Circumference, (circum, around, and fero, 
to carry.) — The line or lines which 
bound any figure; but more especially 
the curved line which bounds the circle, 
and is everywhere equidistant from a 
point within called the centre. If the dia- 
meter of the area be taken as 1, the cir- 
cumference is equal in length to o'1416, 
nearly. 

Circumgyration. — The whirling motion of 
a body about a centre, as of the planets 
about the Sun. 

Circumpolar Stars, {circum, round about.) 



426 



BOUVIER S FAMILIAR ASTRONOMY. 



— Those stars which, owing to the lati- 
tude of the observer, revolve round the 
pole without setting. 

Civil Day.— See Day. 

Civil Month. — See Month. 

Civil Year. — The legal year, which every 
government appoints to be used within 
its own dominions. 

Clamp. — A contrivance used for fixing cer- 
tain parts of an instrument in an im- 
movable position. 

Clavius. — The name of a Lunar mountain. 

Clepsydra, (K-Xerrrw, to steal, and ii(5wp, water.) 
— A kind of water-clock, or an hour-glass, 
serving to measure time by the dropping 
of water from one vessel into another. 
By this instrument the Egyptians mea- 
sured their time and the course of the 
Sun ; and by it, in modern times, Tycho 
Brahe measured the motions of the 
stars. 

Clio. — The twelfth asteroid; discovered by 
Hind, September 13, 1850. 

Clock. — A machine so regulated by the 
uniform motion of a pendulum as to mea- 
sure time with accuracy. The invention 
of the clock is ascribed to Pacificus, arch- 
deacon of Verona, in the ninth century ; 
others attribute the invention to Boethius, 
about the year 510. A common clock 
indicates mean solar time, but clocks 
used in observatories show sidereal time. 

Clock Stars. — Stars which serve to regu- 
late astronomical clocks, their situations 
or places having been very exactly de- 
termined. 

Cluster. — A collection of stars very closely 
congi-egated. Those assemblages which 
require the aid of very powerful tele- 
scopes to resolve or separate them, are 
generally termed clusters. 

Coal-Sack. — A small portion of the heavens 
in the southern hemisphere, near the 
Southern Cross, Avhich, to the naked eye, 
appears entirely devoid of stars; but 
when the telescope is applied, extremely 
small stars are perceptible throughout. 
It has received the name of coal-sack be- 
cause it appears perfectly black when 
contrasted with the adjacent bright por- 
tions of the Milky Way. 

Coincide. — Two surfaces are said to coin- 



cide when their forms agree with each 

•* other throughout. 

Co-Latitude. — The complement of the lati- 
tude, or its difference from 90°. 

CoUimation, {eon, with, and limes, a limit.) 
— The line in a telescope which passes 
through the intersection of the cross-wires 
fixed at the common focus of the lenses. 
The line drawn through the optical cen- 
tres of the eye and the object-lenses. 

CoUimation, Error of. — A deviation of the 
centre wire of a transit instrument from 
its optical axis. — Hind. 

Collimator. — An instrument for ascertain- 
ing the horizontal point, or correcting 
the error of collimation in an instrument. 

Columba. — A southern constellation. Its 
R. A. is bh. 40m., Dec. 35° 0' S. 

Colures, [koXo^, mutilated, and ovpa, a tail; 
so called because a portion of it is always 
below the horizon.) — Two great imagi- 
nary circles intersecting each other at 
right angles at the poles; the one pass- 
ing through the equinoctial, and the 
other through the solstitial, points. 

Colure, Equinoctial. — That colure which 
passes through the equinoctial points 
Aries and Libra. 

Colure, Solstitial. — That colure which 
passes through the solstitial points Can- 
cer and Capricorn. 

Coma, (derived from a Greek word signi- 
fying hair.) — A dense, nebulous cover- 
ing of a comet, called also the Envelope, 
which sometimes renders the nucleus in- 
distinct. The Tail is regarded as an ex- 
pansion or elongation of the coma. — 
Olmsted. 

Coma Berenices. — One of the northern con- 
stellations, the middle of which is R. A. 
\2h. 30w., Dec. 27° 0' N. 

Comes. — An attendant or companion star. 
When two stars appear very near each 
other, the less bright of the two is called 
the comes. 

Comet, (*fo//aco, hairy.) — This name is given 
to bodies belonging to our system with 
highly eccentric orbits, and whose mo- 
tion is not always in the order of the 
signs, but sometimes direct, and at others 
retrograde. They are not confined to 
any portion of the heavens, but may be 



ASTRONOMICAL DICTIONARY. 



427 



seen in all directions. By some early 
writers they were called sTiode stars. 

Cometary Astronomy. — The astronomy of 
comets, embracing all their appearances 
and motions. 

Cometarium. — A machine representing the 
motion of a comet round the Sun. 

Comet-Finder. — A telescope having a large 
object-glass, of large aperture and short 
focal length. 

Cometography. — A history and description 
of comets. 

CommeasuraWe Distance. — A distance 
which can be measured instrumentally, 
or by calculation. 

Communication of Motion. — That act of 
a moving body by which it gives motion, 
or transfers its motion, to another body. 

Commutation, Angle of.— The distance be- 
tween the Sun's true place, as seen from 
the Earth, and the place of a planet re- 
duced to the ecliptic. 

Compass. — A general name given to those 
instruments used to determine the direc- 
tion of the magnetic meridian, and also 
to designate the angular distance between 
that meridian and any horizontal line. 

Compass, Azimuth. — An instrument used 
for taking the bearing of any celestial 
object when it is in or above the horizon, 
that from the magnetic azimuth or am- 
plitude the variation of the needle may 
be known. This compass differs from 
the mariner's compass in having the cir- 
cumference of the card or box divided into 
degrees and quarter degrees. Attached 
to the box is an index with two sights, 
through which the Sun or a star is to be 
viewed at the time of observation. 

Compass, Mariner's. — An instrument used 
at sea to direct and ascertain the course 
of the vessel. It consists of a circular 
brass box, containing a paper card on 
which is marked the thirty-two points 
of the compass. This card is fixed on a 
magnetic needle, so that the point of the 
card marked north is always directed to- 
wards that point of the heavens. The 
invention of the compass is usually as- 
cribed to Flavio Givia, about the year 
1302; it was then divided into eight 
points. By some the invention is as- 



cribed to the Chinese, for their emperor, 
Chiningus, is said to have had a know- 
ledge of it 1120 B. c. The Chinese com- 
pass was then divided into twenty-four 
parts. See Hutton^s Ilathetnatieal Diet., 
art. Compass. 

Compass, Variation. — A compass of very 
delicate construction, intended to indi- 
cate the daily variation of the magnetic 
needle. 

Compass, Variation of.— -The deviation of 
its points from the corresponding points 
in the heavens. When the needle points 
to the east of the true north point of the 
heavens, the variation is east; if it be to 
the west, the variation is west. 

Complement. — That which is wanting to 
complete some certain quantity. 

Complement of an Arc or Angle. — That 
which any given arc or angle wants of 
90°. Thus, the complement of 50° is 40°. 

Composition of Forces, or Motion. — The 
union or assemblage of several forces or 
motions that are oblique to one another, 
into an equivalent one in another direc- 
tion. 

Compound Motion. — That motion which is 
the effect of several conspiring powers or 
forces. 

Compression of tlie Poles.— The amount 
by which the polar is shorter than the 
equatorial diameter, owing to the flatten- 
ing of a planet at its poles. 

Concave. — The inner surface of hollow bo- 
dies, more especially of spherical or cir- 
cular ones. 

Concave Lens. — See Lens. 

Concentric, {con, with, and centrum, a cen- 
tre.) — Having the same centre. 

Cone. — A round pyramid or solid body, 
having a circle for its base, and its sides 
formed by right lines drawn from the 
circumference of the base to a point at 
the top, being the vertex or apex of the 
cone. A right line drawn from the ver- 
tex to the centre of the base is termed 
the axis of the cone. 

Cone of Eays. — The rays of light which 
fall from a luminous point upon a given 
surface, as upon the object-glass of a 
telescope. 

Conic Sections. — That science which treats 



428 



BOUVIER'S FAMILIAR ASTRONOMY. 



of the curved lines and plane figures 
which are produced by the intersection 
of a plane with a cone. 

Conjugate Axis. — See Axis. 

Conjugate Diameter. — The same as conju- 
gate axis. See Axis. 

Conjunction. — The meeting of two hea- 
venly bodies in the same point or place 
in the heavens. 

Conjunction, Apparent.— The situation of 
two bodies being such that a line drawn 
through their centres and produced, 
passes through the eve of the observer, 
but not through the centre of the Earth. 

Conjunction, Inferior. — TThen a planet is 
in inferior conjunction, its geocentric 
longitude is the same as that of the Sun ; 
but the difference between the geocentric 
and heliocentric longitudes equals ISO*'. 

Conjunction, Superior. — The interior pla- 
nets, Mercury and Venus, are in superior 
conjunction when they have the same geo- 
centric longitude as the Sun, but are be- 
yond that luminary as seen from the Earth. 

Conjunction, True. — The situation of two 
bodies being such that a line drawn 
through their centres and produced, wQl 
pass through the centre of the Earth and 
the eye of the observer. 

Consequentia. — In the order of the signs. 

Constant of Aberration.— Light is 8m, 
17"S#. in travelling from the Sun to the 
Earth. In this interval the Earth has 
moved, with her average velocity, through 
an arc of 20-45", which is, therefore, the 
amount of displacement in the Sun's 
longitude, arising from the progressive 
motion of light, and is termed the con- 
stant of aberration. — Hind^ 

Constant Quantities. — Quantities which 
remain invariably the same, while others 
increase or decrease. 

Constellation, (eon, with or together, and 
Stella, a star). — A group or collection of 
stars. The constellations are represented 
on maps and globes by figures of ani- 
mals, <fec. The constellations are divided 
into northern and southern. The north- 
em are those north of the zodiac; the 
southern, those south of it. 

Constellations, Northern. — Those constel- 
lations north of the zodiac 



Constellations, Southern.- Those constel- 
lations south of the zodiac. 

Constellations, Zodiacal. — Those twelve 
constellations situated in the zodiac. 

Contact, [contingo, to touch.) — The relative 
state of two bodies which t^uch each 
other without cutting or entering; the 
parts touching each other are called the 
points of contact. 

Contact, Augle of. — The angle made by a 
curved line and a tangent to it. 

Contact. External. — The nr«f external eon- 
tact is when the disc of a planet appa- 
rently first touches the disc of the Sun ; 
the last external contact is when the 
planet has passed over the Sun, and 
their discs are just about being sepa- 
rated. 

Contact, Internal. — The ^rsf internal con- 
tact is when the disc of the planet is ap- 
parently just perfectly on the Sun's disc; 
and the last internal contact is when the 
planet has passed over the Sun's disc, but 
its edge has not yet projected beyond 
the Sun"s limb. 

Converge, {con, with, and r>€rgo, to incline.) 
— To tend or incline towards the same 
point When two straight lines con- 
verge, they wUl meet if produced sufB- 
ciently. 

Converging Bays. — Those rays which tend 
to a common focus. 

Convex, {convexm, arched.) — Curved and 
protuberant outwards, as the surface of a 
globe or sphere. 

Copemican System. — The solar system as 
it is now understood — that is, the Sun in 
the centre, and the Earth, planets, and 
comets revolving around him. It is so 
called from its founder, Nicholas Coper- 
nicus, who was bom A., d. 1473, at Thorn, 
in Prussia. This system was first pro- 
posed by Pythagoras, and revived by 
Copernicus. 

Copernicus. — The name of a lunar moun- 
tain. 

Cor Caroli. — One of the northern constel- 
lations, situated R. A. 12A. Aim., Dec. 
40° 0' N. 

Cor Hydra. — The chief star in the constel- 
lation Hydra, also known as a Hydrae, or 
A Iphard, 



ASTRONOMICAL DICTIONARY. 



429 



Cor Leonis. — See Regulus. 

Cor Scorpii. — See Antares. 

Corona. — The circle of light often seen in 

5^' total eclipses of the Sun. It is believed 

by some to be the atmosphere of the Sun 

rendered visible by the intervention of 

the Moon. 

Corona Australis. — A southern constella- 
tion. R. A. 18h. 40m., Dec. 40° 0' S. 

Corona Borealis. — A northern constella- 
tion, the middle of which is R. A. 15h. 
25m., Dec. 28° N. 

Corpuscular Theory, (eorpusculum, a small 
body.) — A theory which teaches that all 
^ luminous bodies emit, with extreme ve- 

locity, infinitely small particles, or cor- 
puscles; which are, in fact, portions of 
solid matter, which we call light. 

Corvus. — A southern constellation. R. A. 
12h. 21m., Dec. 20° 0' S. 

Co-sine. — The sine of the complement of 
an angle. 

Cosmical Rising or Setting, (^oct/^oj, the 
loorld, the universe.) — A star rises or sets 
cosmically when it rises or sets when the 
Sun rises. 

Cosmogony. — The science of the formation 
of the universe. 

Cosmolabe, (wff/iOf, world, and XaiiPavoj, to 
take.) — An instrument formerly used for 
measuring angular distances between the 
heavenly bodies. It was also called a 
pantocosm, from i^av, all, and Kocr/xos, 
world. 

Cosmology, (koohos, world, and /.oyog, dis- 
course.) — The science which treats of the 
g.yneral laws by which the physical world 
or universe is governed. 

Cosmometry, [Koa^og, loorld, and fierpov, a 
measure.) — The art of measuring in de- 
grees, the world or sphere. 

Co-tangent. — The tangent of an arc which 
is the complement of another to 90°, or 
what the arc wants of a quadrant. 

Cotidal Lines. — Those lines on the Earth's 
surface connecting those places at which 
it is high or low water, or any other cor- 
responding phases of the tides. 

Course. — The direction in which motion is 
performed. 

Co-versed Sine. — The versed sine of the 
complement of an arc or angle. 



Crater. — A southern constellation, situated 
R.A. llh. 8m., Dec. 15° 0' S. 

Crepusculum or Twilight. — The time from 
the first dawn of morning to the rising 
of the Sun ; and again, between the set- 
ting of the Sun and the last remains of 
day. The boundary line of twilight is 
about 18° from the horizon. 

Crescent, (crcsco, to grow.) — This term is 
applied to the new moon, which, as it re- 
cedes from the Sun, shows a small rim 
of light, which increases till it is full. 
When less than one-half of the illumi- 
nated disc of a heavenly body is visible, 
it is said to present the form of a crescent. 

Crinus. — A name applied to the faint lumi- 
nosity surrounding the head of a comet, 
now generally called the coma. 

Crux. — A splendid southern constellation, 
situated R. A. 127i. 20»»., Dec. 60° S. 

Crystalline Spheres.— Two spheres, sup- 
posed by the ancient astronomers to be 
placed between the Primum Mobile and 
the firmament, and by which they ex- 
plained the motions of the stars, <fec. 

Culminating Point.— That point of the cir- 
cle of a sphere which is on the meridian. 

Culmination, {culmen, the highest point.) — 
The passage of a star or planet over the 
meridian. 

Culmination, Lower. — The passage of a 
circumpolar star across the meridian he- 
low the pole. 

Culmination, TTpper.— The passage of a 
circumpolar star across the meridian 
above the pole. 

Cursa. — A name given to the star in the 
constellation Eridanus,j3alled also /? Eri- 
dani. 

Curtate Distance, {curtains, shortened.) — 
The distance of the place of a body from 
the Sun, reduced to the ecliptic ; or the 
interval between the Sun and that point 
where a perpendicular let fall from the 
body meets the ecliptic. 

Curve, (curvns, bent.) — A line whose direc- 
tion is always changing, in distinction 
from a straight line whose direction is 
always the same at every point. 

Curvilinear. — Relating to curves. Those 
figures formed or bounded by a curved 
line or lines. 



430 



BOUVIER S FAMILIAR ASTRONOMY. 



Cusp, {cuspis, a point.) — The points or horns 
of the Moon or other luminary which as- 
sumes the crescent form. 

Cycle, {kvkXos, a circle.) — A certain period 
of time in which the same phenomena 
return ; a periodical space of time. 

Cycle, Calippic. — A cycle of 76 years, or 
four Metonic cycles. This cycle, a great 
improvement on that of Meton, was first 
proposed by Calippus. 

Cycle of Eclipses. — A period of about 6586 
days, which being the time of a revolu- 
tion of the Moon's node, the eclipses re- 
turn in nearly the same order again. 
This cycle was called by the ancient 
Chaldeans Saros. 

Cycle of Indiction. — See Indiction. 

Cycle, Lunar. — A period of 19 years, in 
which time the new and full moons re- 
turn to the same day of the year, but not 
to the same hour of the day. This is also 
called the Metonic cycle. 

Cycle, Solar. — A period or revolution of 28 
years, at the end of which time the days of 
the month retui-n again to the same days 
of the week. The same order of Leap 
years and of the Dominical letters returns 
to the same days of the week; hence it is 
also called the cycle of the Sunday letter. 

Cyclometry, (kvkXos, a circle, and yLerpzw, to 
measure.) — The art of measuring circles. 

Cygnus. — A beautiful northern constella- 
tion. R. A. 20L 26?3i., Dec. 42° N. 

Cynosura. — A name given by the Greeks 
to Ursa Minor or Little Bear, the star in 
the extremity of the tail being called the 
Pole Star, North Star, oPr Cynosura. 

Dahih. — A star in the constellation Capri- 
cornus, also known as /? Capricorni. 

Daily. — Diurnal. 

Day. — A division of time caused by the 
appearance and disappearance of the Sun. 

Day, Artificial. — The interval of time 
which elapses from the rising to the set- 
ting of the Sun, or the time when the 
Sun is above the sensible horizon. 

Day, Astronomical or Solar. — The term 
of twenty-four hours, beginning when 
the Sun's centre is on the meridian of 
any place, and ending when his centre 
arrives at the same meridian again. 



Thus, 9 o'clock in the morning of Febru- 
ary 14, is called by astronomers February 
13, twenty-one hours. 

Day, Civil. — The time allotted for day, 
which includes one whole rotation of the 
Earth on its axis ; but which begins dif- 
ferently in diflFerent nations. In the 
United States, and in Europe generally, 
the civil day begins at midnight, and 
continues twenty-four hours. 

Day, Mean Solar. — The period of twenty- 
four hours measured by equal motion, as 
that of a clock. 

Day, Sidereal. — The sidereal day is the 
time which elapses between two consecu- 
tive transits of any star at the same me- 
ridian. This measure of time is invari- 
able, not being, as the solar day, affected 
by the yearly revolution of the Earth. 
The sidereal day is Zm. 55-91s. less than 
the mean solar day, 

December, {decern, ten.) — In the Roman 
year this was the tenth month, but is the 
twelfth according to our reckoning. In 
this month the Sun enters Capricorn, 
which is the winter solstice. It was a 
season of festivity among the ancients. 

Dechotomy. — See Dichotomy. 

Decil. — An aspect or position of two pla- 
nets when they are distant from each 
other 36°, or a tenth part of the zodiac. 

Decimal System, {decern, ten,)— That sys- 
tem of calculation in which numbers are 
expressed by tens. 

Declination, {declino, to slope.) — The dis- 
tance of a heavenly body from the equi- 
noctial, either north or south. 

Declination Axis. — That axis of an equa- 
torial telescope which is parallel to the 
plane of the equator. 

Declination, Circles of. — Great circles 
passing through the poles, on which de- 
clination is measured. They are the 
same as meridians in geography. 

Declination of the Compass. — Its devia- 
tion from the true meridian. See Va- 

RIATIOX. 

Declination (Instrumental) of an Object. 

— The angular distance of an object 
from a plane perpendicular to the polar 
axis, and estimated upon the declination 
circle. 



ASTRONOMICAL DICTIONARY. 



431 



Declination, Parallax of. — An arc of the 
circle of declinatioa, by which the pa- 
rallax in altitude increases or diminishes 
the declination of a star. 

Declination, Parallels of. — Lesser circles 
parallel to the equinoctial. The Tropic 
of Cancer is a parallel of declination 23° 
28' north, and the Tropic of Capricorn is 
a parallel of declination the same dis- 
tance south of the equinoctial. 

Declination, Eefraction of. — An arc of 
the circle of declination, by which the 
declination of a star is increased or di- 
minished by means of the refraction. 

Deferent. — See Excentric Cikcle. 

Degree. — The 360th part of a great circle. 
Every degree (marked °) is divided into 
60 minutes, (marked ',) each minute into 
60 seconds, (marked ",) each second into 
thirds, ('",) fourths, ("",) &c. — each term 
being a 60th part of its predecessor. 

Degree of Latitude. — The space or dis- 
tance on the meridian through which an 
observer must move to vary his latitude 
by one degree, or to increase or diminish 
the distance of a star from the zenith by 
one degree. 

Degree of Longitude. — The space between 
two meridians that make an angle of one 
degree with each other at the poles, the 
quantity of which is variable according 
to the latitude, being everywhere as the 
co-sine of the latitude. 

Delphinus. — One of the northern constel- 
lations, the middle of which is situated 
R. A. 20h. 32m., Dec. 12° 0' N. 

Deneb. — Another name for a Cygni, a star 
in the constellation Cygnus. It is also 
called Arided. 

Deneh Algedi. — A name given to a star in 
the tail of Capricornus, also called y Ca- 
pricorn i. 

Deneb Kaitos. — See Diphda. 

Denebola. — A bright star in the constella- 
tion Leo, also called B Leonis. This star 
sometimes receives the appellation of 
Serpha. 

Dense Matter. — Matter whose particles are 
compressed together with a certain de- 
gree of closeness. 

Density, (densitas, closeness.) — That pro- 
perty of bodies by which they contain a 



certain quantity of matter within a cer- 
tain bulk or magnitude. 

Depression of the Horizon. — See Dip of 
THE Horizon. 

Depression of the Pole. — The appearance 
of a receding of the pole when the ob- 
server travels towards the equator, owing 
to the spherical figure of the Earth. 

Depression of the Sun or a Star. — Its dis- 
tance below the horizon ; it is measured 
by an arc of a vertical circle intercepted 
between the horizon and the place of a 
star. 

Descending Constellations. — Those con- 
stellations through which the planets 
descend towards the south, as Cancer, 
Leo, Virgo, &o. 

Descending Node. — That point in a pla- 
net's orbit where it intersects the ecliptic 
proceeding southward. It is marked 
thus 13. 

Descending Signs. — Those signs of the 
zodiac from the Tropic of Cancer to that 
of Capricorn are called the descending 
signs. 

Descension, Oblique. — A point or arc of 
the equator which descends at the same 
time with a star or sign below the hori- 
zon in an oblique sphere. 

Descension, Right. — A point or arc of the 
equator which descends with a star or 
sign below the horizon in a right sphere. 

Dheneb el'Okab. — A star in the constella- 
tion Aquila, known as ^ Aquilae. 

Diagonal, {6ia, through, and y(ji)i'ia, a corner.) 
— A right line drawn across a figure from 
one angle to another; it is sometimes 
called a diameter. 

Diagram. — A drawing or pictorial delinea- 
tion, made for the purpose of demon- 
strating or illustrating some description. 

Dial, {dies, a day.) — An instrument serv- 
ing to measure time by the shadow of 
the Sun. The ancients called it sciathe- 
ricum, because it indicated the hour of 
the day by the shadow. 

Diameter of a Circle, {Sia, through, and 
lierpov, a measure.) — A right line passing 
through the centre of a circle, and ter- 
minated at the circumference on both 
sides. 

Diameter, Apparent.— The angle the true 



432 



BOUVIER S FAMILIAR ASTRONOMY. 



diameter of a heavenly body subtends to 
the eye of the observer. 

Diameter, Conjugate. — That diameter of 
an ellipse which bisects the transverse 
axis. It is the same as the conjugate 
axis. 

Diameter, Equatorial. — A line passing 
through the centre of a sphere and ter- 
minated at the equator; that is, at an 
equal distance from each pole. 

Diameter, Polar. — The line passing through 
the centre of a sphere and terminated by 
the poles. 

Diameter, Transverse.— The longer of the 
two unequal diameters of an ellipse. It 
is also called the greater or transverse 
axis. 

Diameter, True or Eeal. — The distance 
measured by a line passing through the 
centre of a heavenly body and termi- 
nating at its surface. 

Diaphanous. — Translucent A body or me- 
dium through which the rays of light 
may easUy pass, as water, air, glass, talc, 
fine porcelain, <fcc. 

Diaphram. — A species of micrometer. 

Dichotomy. — A term denoting an aspect 
of the Moon when she is in quadrature, 
or shows only one-half of the hemisphere 
which is turned towards the Earth. In 
this situation she is said to be dichoto- 
mized or dichotomoxis. 

Differentiate. — To fix the position of a 
celestial object by comparing it with 
another. 

Digit, {digitus, a finger.) — A measure by 
which eclipses are estimated, being the 
twelfth part of the diameter of a lumi- 
nary. It is an ancient measure, of a 
finger's breadth. If the Sun or Moon 
are said to be six digits eclipsed, it means 
that y^a//the disc is invisible. 

Dione.— The fourth satellite of Saturn. It 
was discovered by Dominic Cassini, in 
March, 1684. 

Diopter or Dioptra. — An instrument in- 
vented by Hipparchus, which served to 
level water-courses, to ascertain the height 
of towers, to determine the magnitudes, 
distances, <fec. of the planets. 

Dioptrics. — That part of optics which treats 
of the effects of light as refracted by pass- 



ing through different media, as air, water, 
glass, and especially the different kinds 
of lenses. 

Dip of the Horizon. — The vertical angle 
contained between a horizontal plane 
passing through the eye of the observer, 
and a line from his eye to the unob- 
structed sensible horizon. 

Diphda. — A star of the third magnitude in 
the constellation Cetus, also known as 3 
Ceti. This star is sometimes called De- 
neb Kaitos, and by the Arabs was named 
Rana Secunda. 

Direct Motion.— See Motion. 

Disc, {ciCTKos, a dish or quoit.) — The body or 
face of the Sun, Moon, or planets. 

Disc, Circle of. — See Circle of the Disc. 

Distance. — The space separating the cen- 
tres of two stars, expressed in seconds 
of arc. 

Diurnal. — Relating to the day. 

Diurnal Aberration. — See Aberration. 

Diurnal Arc. — The arc described by the 
Sun, Moon, or stars, between their rising 
and setting. 

Diurnal Circle. — The apparent circle de- 
scribed by the celestial bodies in conse- 
quence of the rotation of the Earth. 

Diurnal Motion. — See Motiok, 

Diurnal Motion of the Earth. — The rota- 
tion of the Earth round its axis, the du- 
ration of which constitutes the natural 
day. 

Diurnal Motion of a Planet.— The number 
of degrees, minutes, &c. which any pla- 
net moves in twenty-four hours. 

Diurnal Parallax.— See Parallax. 

Diurnal Revolution. — The revolution 
which any celestial body performs on its 
own axis. 

Diverging.— Receding, separating. 

Dodecatemorion. — The twelve signs of the 
zodiac; also the twelfth part of every 
sign. 

Dog. — A name common to two constella- 
tions — the Great and Little Dog : but 
more usually known by the name of Ca- 
nis Major and Canis Minor. 

Dog Days. — See Caxicular Days. 

Dog Star. — The same as Sirius, which see. 

Dominical Letters. — One of the first seven 
letters of the alphabet, otherwise called 



ASTRONOMICAL DICTIONARY. 



433 



the Sunday letter. They are used in 
almanacs to denote the Sundays through- 
out the year. 

Doppelmager. — A spot on the lunar sur- 
face, to which this name is given, 

Dorado or XipMas. — A southern constel- 
lation, situated in R. A. 5h. Om., Dec. 
62° 0' S. 

Double Nebulae. — See Nebula. 

Double Stars. — Two stars connected in one 
system, having a motion round the same 
centre. Double stars can only be dis- 
tinguished by means of a telescope. 

Double Stars, Conspicuous. — Those double 
stars in which both individuals exceed 
the 8*25 magnitude. 

Double Stars, Optical. — Two stars, one of 
which is nearly on a line with the other 
more distant star and the observer's eye, 
but which have no revolution round a 
common centre of gravity, and are not 
members of the same system. 

Double Stars, Physical. — Two stars which 
revolve round the same centre of gravity, 
and which form a binary system. 

Double Stars, Residuary. — Those double 
stars in which both individuals are be- 
low the 8'25 magnitude. 

Draco. — A northern constellation. R. A. 
18^. Om., Dec. 65° N. 

Dubhe. — A star in the constellation Ursa 
Major, termed a Ursa Majoris. It is 
noted as constituting one of the Pointers. 

Dynameter. — An instrument used for ob- 
taining the measure of the magnifying 
power of telescopes. It was invented by 
Ramsden. 

Dynamical Law. — The law which governs 
the motions of moving bodies, or of matter. 

Dynamics. — The science of moving powers, 
or of matter in motion : the motion of 
bodies that mutually act on each other. 

Earth. — The third planet from the Sun; 
the globe on which we live : marked by 
the astronomical character ©. 

Earth-light. — The faint light which is seen 
on the Moon at the time of new moon, and 
is the sunlight reflected from the Earth. 

East. — One of the cardinal points of the 
horizon or of the compass, being the 
middle point between north and south. 



Ebb Tide. — The receding of the sea after 
high tide. See Tides. 

Eccentric. — Those figures, circles, spheres, 
&c. which have not the same centre 
The term is opposed to concentric, which 
signifies having the same centre. 

Eccentric Anomaly.— See Anomaly. 

Eccentric Place in the Ecliptic. — A point 
of the ecliptic to which a planet, as seen 
from the Sun, is referred; or, the place 
of a planet reduced to the ecliptic. See 
Heliocentric Longitude. 

Eccentricity. — The distance between the 
Sun and the centre of a planet's orbit. 

Eclipse, {£K\£nroi, to disappear.) — A priva- 
tion of the light of a luminary by the 
interposition of some opaque body either 
between it and the eye, or between it 
and the Sun. 

Eclipse, Annular. — The total obscuration 
of the Sun, except a luminous ring round 
its edge. 

Eclipse, Central. — An eclipse in which the 
centres of the two luminaries and the 
Earth come into the same straight line. 

Eclipse of the Moon. — The privation of 
the light of the Moon, occasioned by the 
interposition of the Earth. 

Eclipse, Partial. — An eclipse in which only 
a portion of the luminary is obscured. 

Eclipse of the Sun.— An occultation of the 
Sun's body, caused by the interposition 
of the Moon between the Sun and the 
Earth. 

Eclipse, Total. — An eclipse in which the 
whole luminary is darkened. 

Ecliptic, (^kKkciTTTiKdv, so called because the 
eclipses of the Sun and Moon always 
happen under it.) — A great circle of the 
sphere, conceived to pass through the 
middle of the zodiac ; it is the apparent 
path of the Sun, or the real path of the 
Earth. 

Ecliptic Conjunction. — The conjunction 
of the Sun and Moon at the time of new 
moon, both luminaries having then the 
same longitude. 

Ecliptic Limit, Lunar. — If the Moon be 
within 12° of her node at the time of full, 
she will be more or less eclipsed. This 
space of 12° is called the lunar ecliptic 
limit. 



434 



BOUVIER S FAMILIAR ASTRONOMY. 



Ecliptic Limit, Solar. — If the Moon be 
within 17° of her node at the time of new 
moon, she will eclipse the Sun; hence 
this space of 17° is called the solar eclip- 
tic limit. 

Ecliptic, Obliquity of. — The angle or in- 
clination of the Earth's equator to the 
plane of her orbit. 

Egeria. — The thirteenth asteroid, disco- 
vered by De Gasparis, November 2, 1850. 

Egress. — The passing off of an inferior 
planet after a transit over the Sun's disc. 

Egyptian System. — That system of the 
universe as taught by the Egyptians. 

Electra. — A name given to one of the Plei- 
ades. 

Elements. — Those principles deduced from 
astronomical observations and calcula- 
tions, and those fundamental numbers 
which are employed in the construction 
of tables of the planetary motions. The 
elements of a planet's orbit are seven in 
number. 

Elements of an Orbit. — These are the mean 
distance, eccentricity, longitude of the 
perihelion, periodic time, longitude of 
the ascending node, inclination of the 
orbit, and longitude of the epoch. 

Elevation of the Equator. — The height of 
the equator above the horizon. The ele- 
vation of the equator and of the pole 
taken together equals 90°. The eleva- 
tion of the pole being found and sub- 
tracted from 90° leaves the elevation of 
the equator. 

Elevation of tbe Pole. — The angle or arc 
of the meridian intercepted between the 
horizon and the pole. It is always equal 
to the latitude of the place. 

Elevation of a Star. — Its angular height 
above the horizon ; or an arc of the ver- 
tical circle intercepted between the star 
and the horizon. 

Ellipse. — One of the conic sections, popu- 
larly called an oval. It was first called 
an ellipse or ellipsis by Appolonius, the 
first and principal author of the conic 
sections. 

Ellipticity. — Eccentricity, or deviation 
from the circular or spherical form. In 
the terrestrial spheroid, it is the ratio or 
difference between the two semi-axes. 



El Natb. — A bright star in the constella- 
tion Taurus, known also as /? Tauri. 

Elongation. — The angular distance of a 
planet east or west of the Sun. This 
term is generally applied to the interior 
planets. 

Elongation, Angle of. — An angle con- 
tained under lines supposed to be drawn 
from the centres of the Sun and a planet 
to the centre of the Earth. 

El Riscba. — The Arabic name for the star 
known as a Piscium, in the constellation 
Pisces. It was also called Okdah. The 
Greeks called this star Syndesmos. 

Embolimseus. — Intercalary; a term used 
for that of Bissextile or Leap year. 

Emersion. — The reappearance of the Sun, 
Moon, planet, or star, after having suf- 
fered an eclipse. 

Enceladus. — The second satellite of Sa- 
turn ; it was discovered by Sir W. Her- 
schel, August 19, 1787. 

Engonasis. — The same as Hercules. See 
Hercules. 

Enif. — Another name for c Pegasi, a star 
in the constellation Pegasus. This star 
is sometimes called Enir. 

Envelope. — A term applied to that nebu- 
lous covering of the head of a comet also 
known by the name of coma. 

Epact. — The excess of the solar month 
above the lunar synodical month ; or of 
the solar year above the lunar year of- 
twelve synodical months. 

Epbemeris, {nixkpa, a day.) — Tables calcu- 
lated by astronomers, showing the state 
of the heavens for every day at noon ; 
that is, the places of all the planets, &c. 
for each day. 

Epicycle. — In the Ptolemaic astronomy, a 
small circle whose centre is in the cir- 
cumference of a greater, which, being 
fixed in the deferent of a planet, is car- 
ried along with it, and j^et with its own 
peculiar motion carries the body of the 
planet attached to it round its proper 
centre. — Ptol. Almag. 1. iii. c. 3. 

Epocb. — A fixed point of time, from whence 
succeeding years are numbered or reck- 
oned. Different nations have different 
epochs. The Christians chiefly use the 
Epoch of the Nativity of Christ; the Mo- 



ASTRONOMICAL DICTIONARY. 



435 



hammedans, the Hegira; the Jews, the 
Creation of the World, or the Deluge; 
the ancient Greeks, the Olympiads ; the 
Romans, the building of their city, &c. 
Also, the time to which certain given 
numbers or quantities apply. 

Epoch of the Mean Longitude.— The mean 
longitude of a planet computed for a 
fixed date. This is also called the longi- 
tude of the epoch. 

Equable Motion. — See Motion. 

Equation of the Centre. — The diflFerence 
between the true and mean place of a 
planet. It is also called Prosthaphe- 
resis. 

Equation of Equinoxes. — The diflFerence 
between the mean and apparent places 
of the equinox. 

Equation of the Equinoxes in Eight As- 
cension. — The distance from the reduced 
place of the mean equinox to the appa- 
rent equinox. 

Equation, Personal. — A personal equation 
is the difference of time with which two 
observers note the transit of a star over 
each of the wires of an instrument. 

Equation of Time. — The diflFerence between 
mean and apparent time, or the diflFer- 
ence between the Sun's mean motion and 
his right ascension. 

Equation of Sidereal Time.— The diflFer- 
ence between apparent and mean side- 
real time. 

Equator. — A great circle of the Earth, 
equally distant from its two poles. It is 
sometimes, by seamen, called the Line. 

Equatorial Diameter. — The diameter of a 
sphere measured through the centre and 
terminated both ways by points on the 
surface situated at equal djstances from 
both poles. 

Equatorial Horizontal Parallax. — See Pa- 
rallax. 

Equatorial Telescope. — A telescope so ad- 
justed as to have two axes of motion at 
right angles to each other, one axis be- 
ing parallel to the axfs of the Earth, 
the other parallel to the plane of the 
equator. The motion which is parallel 
to the plane of the equator being per- 
fectly uniform, when a star is kept con- 
stantly in the field of view, can be per- 



formed by means of clock-work attached 
to the instrument. 

Equi-angular. — Having equal angles. 

Equilateral, {cequabilis, of the same dimen- 
sions, and latera, side.) — Having equal 
sides. Triangles which have all their 
sides equal are said to be equilateral. 

Equilibrium, (cequus, equal, and libra, a 
balance.) — An equality between two 
forces acting in opposite directions; an 
equality of weight. 

Equinoctial. — A great circle in the heavens 
under which the equator moves in its 
diurnal motion : the plane of the equator 
extended to the heavens. 

Equinoctial Colure. — A great circle pass- 
ing through the poles of the heavens and 
the equinoctial points. 

Equinoctial Points.— Those points where 
the equator and ecliptic intersect each 
other. 

Equinoctial Time. — See Tisie. 

Equinoxes, (cequus, equal, and nox, night; 
because then the days and nights axe 
equal.) — The time when the Sun enters 
the equinoctial points. Vernal equinox 
is about the 21st of March ; the autumnal 
equinox about the 22d of September. 

Equuleus. — A northern constellation, the 
middle of which is R. A. 217i. 4»?., Dec. 
5° 0' N. 

Equuleus Pictorius. — A southern constel- 
lation, the middle of which is R. A. 5h. 
30»i., Dec. 52° 0' S. 

Era. — A fixed point of time ; an epoch. 
Also, a way or mode of counting time. 

Era, Christian. — The computation of time 
from the birth of Christ, which was not 
introduced till the sixth century, in the 
reign of Justinian. Before that period 
time was computed from the Olympiads, 
the year of Rome, &g. 

Era of Nabonassar. — An epoch of time 
reckoned from B. c. 747 ; it is of import- 
ance in astronomical chronology. 

Eratosthenes. — The name of a lunar moun- 
tain. 

Eridanus. — A southern constellation, the 
middle of which is R. A. 4/;. Oh;., Dec. 
10° S. 

Errai. — A star in the constellation Cepheus, 
known as y Cephei. 



436 



BOUVIER S FAMILIAR ASTRONOMY. 



Erratic. — A term applied to the planets, 
which are called erratic or wanderinor 



Error of the Clock. — The error of the clock 
is its difference from true sidereal time. 

Establishment of the Port.— The differ- 
ence between the actual and theoretical 
time of high water at any place. 

Etamin. — A star situated in the constella- 
tion Draco, also known as y Draconis. 

Eunomia. — The fifteenth asteroid; it was 
discovered by Grasparis, July 29, 1851. 

Euphrosyne. — The thirty-first asteroid ; it 
was discovered by Ferguson, September 
1, 1854. 

Euterpe. — The twenty-seventh asteroid; it 
was discovered by Hind, Nov. 8, 1853. 

Evection. — The Libration of the Moon ; an 
inequality in her motion, caused by a 
change in the eccentricity of her orbit, 
whereby her mean longitude is sometimes 
increased or diminished to the amount 
of 1° 20'. 

Excentric Circle. — In the ancient Ptole- 
maic astronomy, was the orbit which the 
planet was supposed to describe about 
the Earth, and which was supposed to 
be eccentric with it. It was also called 
the deferent. 

Excentricity. — The deviation of an ellip- 
tical orbit from a circle. It is usually 
expressed in parts of the mean distance 
of a planet or comet. See Angle of Ec- 
centricity. 

Excursion. — Another word for elongation. 

Exterior Angle.— See Angle. 

Exterior Planets. — Those planets whose 
orbits are beyond the orbit of the 
Earth. 

Eye-piece, Diagonal. — A flat piece of po- 
lished metal placed between the two 
lenses of the eye-piece at an angle of 45° ; 
a rectangular prism of glass is often used 
instead of the metal. 

Eye-piece, Negative. — The negative eye- 
piece is formed of two plano-convex lenses 
placed with their curved faces towards 
the object-glass. 

Eye-piece, Positive. — The positive eye- 
piece is formed of two plano-convex 
lenses, having their curved faces turned 
towards each other. 



Eye-piece of a telescope.— A microscope, 
composed of two or more lenses, applied 
to the eye-end of the telescope. 

Face. — The plane surface of any solid. 

Faculae, [facula, a small torch.) — Spots on 
the Sun, more luminous than the other 
parts of his surface. 

Falcated, {falx, a sickle, or reaping-hook.) 
— One of the phases of the Moon or pla- 
nets, otherwise called horned. When 
the enlightened part appears in the form 
of a crescent or sickle, it is said to be 
falcated. 

Falling Stars. — See Meteor. 

Fasciae. — See Belts. 

February, (so called from Februa, a Ro- 
man feast held in that month.) — The 
second month of the year, containing 28 
days for three years, and every fourth 
year 29 days. In the first ages of Rome 
February was the last month in the year, 
and preceded January till the Decemviri 
made an order to place it after January. 
See Bissextile. 

Field of View.— That portion of the hea- 
vens visible at one time through a tele- 
scope. The fields of telescopes differ ac- 
cording to their powers. 

Figure. — In geometry, a diagram or draw- 
ing representing a magnitude upon a 
plane surface. 

Figure of the Earth. — The shape or form 
of the globe on which we live ; it is found 
to be an oblate spheroid, the axis of which 
coincides with the axis of the Earth. 
The terrestrial elements, as found by the 
United States Corps of Topographical 
Engineers, are as follows : 
Equatorial radius, 6377397-15 metres. 
Polar " 6356078-96 « 

Eccen. of meridian, 0-0816967. 

The metre employed in these elements 
is taken equal to 39-36850154 American 
standard inches, as determined by Hassler 
in 1832. These elements are those at 
present used upon the coast survey. Re- 
duced to yards, the elements are — 

Equatorial radius = 6974532339 yards. 

Polar " = 6951218-059 " 

Eccen. of meridian = 0-0816967. 

Ellipticity = 1 _ 

80966 



ASTRONOMICAL DICTIONARY. 



437 



The following are the elements of the 
Earth's figure as given by Bessel and 
Airy: 

Bessel. 

Feet. Miles. 

Eq. radius, 20923596 = 3962-802 

Polar " 20853662 = 3949-557 

EUipticity, _J_ 

299-16 

Airy. 

Feet. Miles. 

Eq. radius, 20923713 = 3962-824 

Polar " 20853810 = 3949-585 

EUipticity, i 

299-33 

Davies's and Peck's Ilath. Diet, 

Filar-micrometer. — See Micrometer. 

Finder. — A small telescope, having a low 
power and a large field of view, attached 
to the larger telescope. The finder and 
the large telescope should have their 
axes parallel to each other. 

Firmament. — The sphere of the fixed stars. 

First Quarter. — When the Moon is 90° 
distant from the Sun after new moon, she 
is in her first quarter. 

Fixed Stars. — Those bodies which do not 
belong to the solar system. They gene- 
rally retain the same position and dis- 
tance with respect to each other. 

Flora. — The eighth asteroid ; it was disco- 
covered by Hind, October 18, 1847. 

Focal Distance. — The distance from the 
focus of an ellipse to the nearest extre- 
mity of the transverse axis. 

Focal Length. — The distance of the image 
of the object from the object-glass is 
called the focal length of the telescope. 
This is commonly a little greater than 
the length of the main tube. 

Foci of an Ellipse. — Two points in the 
major axis, such that the sum of the two 
lines drawn from them to any point in 
the ellipse is equal to the major axis. 

Focus. — That point in which several rays 
meet after having been either reflected 
or refracted. 

Focus of a Mirror. — The focus of a con- 
cave mirror is a point distant from its 
surface equal to one-half of the radius of 
the sphere of which the mirror is an arc. 

Following. — See North Following. 

Fomalhaut— A star of the first magnitude 
in the constellation Piscis Australis. It 
is also known as a Piscis Australis. 



Force. — That which is the cause of motion ; 
power. 

Force, Accelerative. — That power which 
accelerates the velocity of motion. 

Force, Motive. — Otherwise called momen- 
tum, which see. 

Force, Retardive. — That power which re- 
tards the velocity of motion. 

ForestaflF. — -An instrument formerly used 
to take the altitudes of the heavenly 
bodies. With the forestaff the observer 
stood with his face to the object observed, 
while with the baeJcstaff the back of the 
observer was turned to the object. 

Formed Stars. — Those stars which are ar- 
ranged under certain figures called con- 
stellations. 

Fornax Chemica. — A southern constella- 
tion, the middle of which is R. A. 3A. 
Om., Dec. 30° S. 

Fortuna. — The nineteenth asteroid ; it was 
discovered by Hind, August 22, 1852. 

Frigid Zone.— The space of 23° 28' about 
each pole. The Arctic Circle is the 
boundary of the north frigid zone, and 
the Antarctic Circle the boundary of the 
south frigid zone. 

Frustrum. — A part of some solid body sepa- 
rated from the rest. The frustrum of a 
cone is that part which remains when the 
top is cut oflf by a plane parallel to the 
base : it is otherwise called a truncated 
cone. 

Galaxy, (yaXa(croj, milk.) — A whitish, lumi- 
nous band, of irregular form, which is 
seen on a dark night stretching across 
the heavens. It contains myriads of 
stars, so crowded together that their 
united light only reaches the unassisted 
eye. It is also known by the names of 
the Milky Way or Via Laetea. 

Gassendi. — A spot on the surface of the 
Moon known by this name. 

Gauging. — The estimation of the number 
of visible stars in any field of view of a 
telescope ; a term used by Sir W. Her- 
schel to signify the penetration into space 
with instrumental power. 

Gemini. — One of the zodiacal constella- 
tions, the middle of which is R. A. 7/*. 
20;«., Dec. 25° 0' N. 



438 



BOUVIER S FAMILIAR ASTRONOMY. 



Gemma- — See Alphecca. 

Geocentric. — Having the Earth for the cen- 
tre. Properly speaking, the Moon alone 
is geocentric. 

Geocentric Diameter. — The diameters of 
the Sun, Moon, or planets, as seen from 
the centre of the Earth. It is the same 
as vertical diameter. 

Geocentric Latitude. — See Latitude. 

Geocentric Longitude. — See Longitude. 

Geocentric Parallax. — The apparent 
change of place of a body arising from a 
change of a spectator's station from the 
surface to the centre of the Earth. 

Geocentric Place. — The place of a planet 
as it appears from the Earth; a point in 
the ecliptic to which a planet, seen from 
the Earth, is referred. 

Geodesical Determination of the Situa- 
tion of two Observatories, or other 
Points, with respect to each other.— 
The determination of their respective 
situations by means of triangular mea- 
surements. See Geodesy. 

Geodesy. — That part of geometry which 
has for its object the determination of the 
magnitude and figure of the Earth, or any 
portion of its surface; land surveying. 

Georgium Sidus. — See Uranus. 

Ghost. — The image of a dot made to fall 
on a certain point or stroke of a gra- 
duated instrument. This image is cre- 
ated by means of an optical contrivance, 
the invention of Mr. Ramsden. 

Giauzar. — A star in the constellation Draco, 
called also X Draconis. 

Gibbous. — The Moon is said to be gibhoiis 
when more than the half, and less than 
the whole, of her enlightened disc is visi- 
ble to us. The same term is applied to 
the planets Mercury, Venus, and Mars, 
when presenting the same appearance. 

Giedi. — The name of a quintuple star in 
the constellation Capricornus, also known 
as a Capricorni. 

Gienah. — A star in the constellation Cyg- 
nus, also called e Cygni. 

Globe. — A round or spherical body, more 
usually called a sphere, bounded by one 
uniform convex surface, every point of 
which is equally distant from a point 
within, called its centre. 



Globe, Artificial. — Artificial globes are 
either terrestrial or celestial. The ter- 
restrial globe is made of paper, paste- 
board, <&c., and on the surface is a map 
representing the various portions of the 
Earth's surface. The celestial globe is 
a map or representation of the concave 
surface of the heavens, with the constel- 
lations and principal fixed stars. 

Gnomon. — An instrument used by the 
ancients in astronomical observations. 
It consisted of a perpendicular staff" 
or pillar, the shadow of which indi- 
cated the altitude of the Sun. The 
term is now applied to the finger of a 
sundial. 

Golden Number. — See Cycle of the Moon, 
or Metonic Cycle. 

Gomeisa. — A star in the constellation Ca- 
nis Minor, generally known as /? Canis 
Minoris. This star was also called Al- 
dlirzam by the Arabs. 

Goniometry, (yui/ta, angle, and [isrpoVf a 
measure.) — The art of measuring angles, 
either on the surface of the Earth or 
upon any plane surface. 

Graduation. — The art of dividing a scale 
or arc of a circle into any number of 
equal parts. 

Graffias. — See Akrab. 

Graphometer, {Ypa(pco, to describe, and/isrpov, 
a measure.) — An instrument used for 
measuring angles whose vertices are at 
its centre. 

Gravitation, or Universal Gravity. — That 
power by which all the bodies and par- 
ticles of matter in the universe tend to- 
wards one another. 

Gravity, Centre of. — See Centre. 

Great Bear. — See Ursa Major. 

Great Circles. — See Circle. 

Gregorian Calendar. — The calendar used 
at the present time, commenced by Pope 
Gregory XIII. in 15S2. 

Gregorian Epoch. — The epoch or time 
from which the Gregorian calendar or 
computation took place. 

Gregorian Telescope. — An instrument in- 
vented by James Gregory. See Tele- 
scope. 

Gregorian Year. — The year reckoned ac- 
cording to the new style introduced by 



ASTRONOMICAL DICTIOXARY. 



439 



Pope Gregory in the year 1582, and 
from whom it took its name. 

Grus. — A southern constellation, the mid- 
dle of which is R. A. 22h. Ow., Dec. 
45° S. 

Guards. — This term is applied to the stars 
/? and y in the constellation Ursa Minor. 

Halo.— See Coboxa. 

Hamel. — A star called also a Arietis, situ- 
ated in the constellation Aries. 

Harvest Moon. — The full moon which oc- 
curs nearest to the autumnal Equinox. 

Head. — The nucleus and coma of a comet 
are frequently included under the general 
term head. 

Heavens. — The blue vault of the sky in 
which the heavenly bodies are situated. 

Hebe. — The sixth asteroid; it was disco- 
vered by Hencke, July 1, 1847. 

Heka. — A name given to a star in the 
constellation Orion, generally called X 
Orionis. 

Heliacal, (HXtof, the Sun.) — Pertaining to 
the Sun. 

Heliacal Rising and Setting of a Star. — 
The rising when the Sun rises, and set- 
ting when he sets. 

Heliocentric. — As seen fi-om, or having re- 
lation to, the centre of the Sun. 

Heliocentric Latitudes and Longitudes. — 
The latitudes and longitudes of the pla- 
nets as they would appear if seen from 
the Sun. 

Heliocentric Parallax. — The arc of a great 
circle of the celestial sphere drawn from 
the heliocentric to the geocentric place 
of a body. It is the path a body would 
appear to describe to an observer who 
would travel from one extremity to the 
other of the Earth's radius vector. 

Heliocentric Place of a Planet. — The 
place a planet would appear to occupy 
if viewed from the Sun ; or the point of 
the ecliptic in which a planet would 
appear to be if seen from that lumi- 
nary. 

Heliometer, (HXioj, the Sun, and nerpov, a 
measure.) — An instrument used for mea- 
suring the diameter of the Sun, Moon, 
and planets. 

Heliostat, («X(oj, the Sun, and ffraroj, to Htand 



still.) — An instrument so contrived that 
a reflected ray of light may be retained 
in a fixed position, notwithstanding the 
apparent motion of the Sun. 

Hemisphere, (H//(, « half.) — Half a globe 
or sphere ; every great circle divides the 
globe into two hemispheres. 

Hercules. — One of the northern constella- 
tions, the middle of which is R. A. 16A. 
4Sm., Dec. 30° 0' N. 

Herschel. — See Uranus. 

Heteroscii, (from 'irzpog, another, and cKia, 
a shadotv.) — All the inhabitants of the 
Earth, except those of the torrid zone, 
are called heteroscii, because their sha- 
dows at noon are always projected the 
same way ; that is, towards the poles. 

High Water. — That state of the tides when 
they have flowed to their greatest height. 

Higher Apsis. — See Apsis. 

Hircus. — See Capella. 

Homam. — A star in the constellation Pe- 
gasus, called also ^ Pegasi. 

Horary, {hora, an hour.) — Relating to 
hours. 

Horary Circles. — Those circles on the ar- 
tificial globe which are drawn at the dis- 
tance of an hour from each other, to 
mark the hours. 

Horary Motion. — The motion or space 
moved by a heavenly body in an hour. 

Horizon, (opt^w, to hound.) — The boundary 
to the view on all sides. The horizon is 
either real or rational, or sensible or ap- 
parent. 

Horizon, Artificial. — An instrument hav- 
ing a reflecting plane situated at right 
angles to a perpendicular line. This is 
also called a. jjortnble horizon. 

Horizon, Poles of. — The zenith and nadir 
are the two poles of the horizon, each 
being 90° distant from it. 

Horizon, Portable. — See Horizox Arti- 
ficial. 

Horizon, Real or Rational. — That horizon 
whose plane is supposed to pass through 
the centre of the Earth ; and which, 
therefore, divides the globe into two equal 
portions or hemispheres. 

Horizon, Sensible or Apparent. — That cir- 
cle which divides the visible part of the 
Earth and heavens from that which is 



I 



440 



bouvier's familiar astronomy. 



invisible; the circle which bounds the 
view on all sides. 

Horizontal. — Level or parallel with the 
horizon. 

Horizontal Distance. — That distance of 
two objects from each other estimated in 
the plane of the horizon. 

Horizontal Parallax.— A parallax which 
is in the plane of the horizon. See Pa- 
rallax. 

Horizontal Plane. — A plane parallel to the 
horizon of a given place. 

Horizontal Point. — That point on the limb 
of a graduated circle coincident with the 
plane of the horizon. The chief points 
of the horizon are the north, south, east, 
and west points. 

Horizontality. — The horizontality of the 
axis of a transit instrument is the adjust- 
ment by means of the spirit-level, caus- 
ing the axis of the instrument to be ex- 
actly in the plane of the horizon. 

Horns of the Moon. — The Moon is said to 
be horned when her visible enlightened 
disc is in the form of a crescent. The 
points or extremities of the crescent are 
called the horns. 

Horologium. — A southern constellation, 
the middle of which is situated R. A. 2h. 
40m., Dec. 55° 0' S. 

Hour, (ojpa, hour, which is derived from the 
Egyptian Horus, a name given to the 
Sun, as being the father of Hours.) — It 
is the twenty-fourth part of a natural 
day. The Egyptians, and afterwards the 
Greeks, divided their day into twelve 
hours ; this is the most ancient division 
of the day. 

Hour Angle (Instrumental) of an Object. 
— The angular distance of an object from 
a vertical plane passing through the 
polar axis, and estimated upon the hour 
circle. 

Hour Circle. — See Circle. 

Hour, Sidereal. — The twenty-fourth part 
of the interval of time between two con- 
secutive passages of a fixed star over the 
meridian of a place. 

Hour, Solar. — The twenty-fourth part of 
the time between two consecutive pas- 
sages of the Sun over the meridian of a 
place. 



Humboldt. — The name of a mountain on 
the Moon. 

Hyades. — A cluster of stars in the constel- 
lation Taurus. The ancients supposed 
their rising and setting to be attended by 
rain or storms : thus, Horace calls them 
tristes hyades ; Virgil, pluvias hyades ; 
and Claudian, nimhosas hyades. 

Hyadum Primus. — A star in the group of 
the Hyades, and belonging to the constel- 
lation Taurus. It is also called y Tauri. 

Hyadum Secundus. — A star in the con- 
stellation Taurus, and group of the Hy- 
ades, also known as 6 Tauri. 

Hydra. — A southern constellation, the mid- 
dle of which is situated R. A. 10^. 40m., 
Dec. 13° S. 

Hydrus. — A southern constellation, the 
middle of which is R. A. Ih. 52m., Dec. 
66° 0' S. 

Hygeia. — The tenth asteroid; it was dis- 
covered in the year 1849, by Gasparis. 

Hyperion. — The seventh satellite of Sa- 
turn ; it was discovered by Bond, and 
afterwards by Lassel, in September, 1848. 

Immersion. — When a satellite suffers 
eclipse, the act of passing into the sha- 
dow, or disappearing behind the pri- 
mary, is called immersion. Also, when 
a star or planet comes so near the Sun 
as to be obscured by his rays. 

Impetus. — Force, momentum. 

Incidence or Line of Incidence. — The di- 
rection or inclination in which one body 
strikes or acts on another. 

Incidence, Angle of.— The angle compre- 
hended between the line of incidence and 
a perpendicular to the body acted upon. 

Inclination. — The angle which one plane 
makes with another. 

Inclination of the Axis. — The angle which 
the axis of a planet makes with its orbit. 

Inclination of an Orbit. — The angle which 
the orbit makes with the ecliptic. 

Incommeasurable Distance. — A distance 
which cannot be determined either by 
direct measurement or by calculation. 

Index of Kefraction. — The constant ratio 
which exists between the sines of the 
angles of incidence and refraction. 

Indiction. — An epoch or manner of count- 



ASTRONOMICAL DICTIONARY. 



441 



ing time among the Romans, whicli was 
introducevi by the Emperor Constantino 
in place of the Olympiads. It contains 
a cycle of 15 years. To find the indic- 
tion, add 3 to the given year of Christ, 
divide the sum by 15, and the remainder 
will be the year of the indiction. If there 
be no remainder, the indiction is 15. 

Indus. — A southern constellation, the mid- 
dle of which is in R. A. 21h, 20m., Dec. 
60° 0' S. 

Inequalities of the Moon. — Certain irre- 
gularities of the Moon's motion, caused 
by the combined attraction of the Sun 
and Earth, the principal of which are 
equation of the centre, evection, variation, 
and annual equation. 

Inequalities, Secular. — Certain small ir- 
regularities in the motions of the planets, 
which become important only after the 
lapse of centuries. 

Inequality, Great. — The variation in the 
orbital positions of Jupiter and Saturn, 
caused by their disturbing action on each 
other. It passes through all its changes 
of magnitude in about 918 years. 

Inertia. — See Vis Inertia. 

Inferior Conjunction. — The conjunction 
of the planets Mercury or Venus with 
the Sun at the time they are between 
that luminary and the Earth. 

Inferior Meridian. — That half of the me- 
ridian below the horizon. 

Inferior Planets. — The same as the inte- 
rior planets. 

Informed Stars.— See Sporades. 

Ingress. — The Sun's entrance into one of 
the signs. In transits of Venus or Mer- 
cury it is the entrance of the planet on 
the Sun's disc. 

Intensity of Light.— The degree of bril- 
liancy of a planet or comet expressed as 
a number varying with the distance of 
the body from the Sun and Earth. 

Irene. — The fourteenth asteroid ; it was 
discovered by Hind, May 19, 1851. 

Iris. — The seventh asteroid ; it was disco- 
vered by Hind, August 13, 1847. 

Irresolvable Nebulae.- Nebulae which ap- 
pear to be too distant for our telescopes 
to resolve into stars, but which have a 
faint cloud-like appearance. 



Intercalary. — A term signifying any por- 
tion of time added to the year to adjust 
it to the course of the Sun : thus, the odd 
day inserted in Leap year is called the 
intercalary day. 

Interior Planets. — Those planets whose 
orbits are within that of the Earth. 

Izar. — A star in the constellation Bootes, 
also called e Bootis. 

January. — The first month in the year. 
So called by the Romans from Janus, 
one of their divinities, who had two faces ; 
because on the one side the first day of 
this month looked towards the new year, 
and on the other towards the old one. 
Others think it derives its name from 
Janua, a gate or door, it being the open- 
ing of the year. 

Japetus. — The eighth satellite of Saturn; 
it was discovered by Dominic Cassini in 
October, 1671. 

Jovicentric. — As viewed from, or relating 
to, the centre of Jupiter. 

Julian Calendar. — That depending on the 
Julian year and account of time; it is 
so called from Julius Caesar, by whom it 
was established. 

Julian Epoch. — That period of the Julian 
calendar which began the forty-sixth 
year before Christ. 

Julian Period.— A cycle of 7980 years, in- 
vented by Scaliger. It is formed by 
multiplying together the cycles of the 
Sun, Moon, and the indiction. The Ju- 
lian period dates from the year 4713 b. c. 

Julian Style. — The mode of reckoning in- 
stituted by Julius Caesar, in which every 
year divisible by 4 without a remainder 
consisted of 366 days, and all others of 
365. The same as Old Style. 

June. — The fourth month in the old Roman 
year, and the sixth in our year. By the 
ancient Romans it was called Junius, in 
honor of the youth of Rome who served 
then in war. Some suppose it to be 
called in honor of Junius Brutus. On the 
21st of this month the Sun enters Cancer. 

Juno. — The third asteroid. 

July. — The seventh mouth in the year. It 
was so named by Mark Antony, in honor 
of Julius Ciesar, who was born in this 



442 



BOUVIER S FAMILIAR ASTROXO.MY. 



month. On the 21st day of this month 
the Sun enters Leo. 

Juno. — One of the asteroids, discovered by- 
Harding, September 1, 1804. It is the 
third in order of discovery. 

Jupiter. — The largest known planet of our 
system, situated next within the orbit of 
Saturn. 

Kaus. — The hoio. This name is given to 
the stars <5 e and X in the constellation 
Sagittarius. 

Kalpeny. — A star in the constellation Aqua- 
rius, marked /? Aquarii on the catalogues. 
It is also called Sa'd-as-suud. 

Kaphra. — A star in the constellation Ursa 
Major, also called k Ursae Majoris. 

Keid. — The name of 40 Eridani, a star in 
the constellation Eridanus. 

Kepler. — The name of a lunar mountain. 

Kepler's Laws. — Those laws of the planet- 
ary motions discovered by Kepler. The 
first two were announced by their author 
in 1609, and the third in 1618. 

Kitalpha. — A name given to the star in 
the constellation Equiileus, known also 
as a Equiilei. 

Kochab. — An Arabic name given to the 
star Urs^ Minoris, in the constellation 
Ursa Minor. 

Korneforos. — A bright star in the constel- 
lation Hercules, also known as /? Her- 
culis. 

Lacerta. — A northern constellation, the 
middle of which is R. A. 22h. 28m., Dec. 
45° 0' N. 

Last Quarter. — When the Moon is 90° dis- 
tant from the Sun, after full moon, she 
is in her last quarter. 

Latitude, Geocentric. — The angle included 
between the radius of the Earth through 
the place on its surface, and the plane 
of the equator. The geocentric latitude 
is always a little less than the geographic 
latitude. 

Latitude, Heliocentric— The latitude of 
a star or planet as seen from the Sun. 

Latitude of the Moon, North Ascending. 
— "When the Moon proceeds from the as- 
cending node towards her northern limit. 

Latitude of the Moon, North Descending. 



— When the Moon returns from her 
northern limit towards her descending 
node. 

Latitude of the Moon, South Ascending. 
— When the Moon returns from her 
southern limit towards her ascending 
node. 

Latitude of the Moon, South Descending. 
— When the Moon proceeds from her de- 
cending node towards her southern limit. 

Latitude of a place on the Earth. — The 
distance of a place from the equator, 
reckoned on an arc of the meridian to- 
wards the north or south pole. 

Latitude, Reduction of. — The difference 
between the latitude and the central lati- 
tude. 

Latitude of a Star or Planet.— Its distance 
from the ecliptic, reckoned from thence 
north or south towards the poles of the 
ecliptic. 

Lagging of the Tides.— See Tides. 

Leap Year. — See Bissextile. 

Lens. — A transparent substance of a differ- 
ent density from the surrounding me- 
dium, having its two surfaces so formed 
that the rays of light in passing through 
it shall have their direction changed. 
Lenses are of two kinds — viz. : coyivex and 
concave. Convex lenses are always thicker 
in the middle and thinner at the edges : 
and concave lenses are thinner in the 
middle and thickest at the edges. Con- 
vex lenses are of two kinds — plano-con- 
vex, having one surface plane or flat and 
the other spherical or convex ; and double 
convex, having both sides 'spherical or 
convex. Concave lenses are also of two 
kinds — namely, plano-concave, having 
one side plane and the other surface 
concave; and double concave, having both 
surfaces concave. The meniscus lens is 
convex on one surface and coocave on 
the other; the concavo-convex lens is, like 
the meniscus, concave on one surface and 
convex on the other, but the convex sur- 
face is the arc of a smaller sphere than 
the concave side. Lenses are usually 
formed of glass ; but they may be made 
of diamond, rock, crystal, or fluids of 
different densities and refractive powers, 
enclosed between glass of a proper form. 



ASTRONOMICAL DICTIONARY. 



I,eo. — One of the zodiacal constellations, 
the middle of which is in R. A. lOh. 20m., 
Dec. 15° N. 

Leo Minor. — A northern constellation, the 
middle of which is situated in R. A. 10/t. 
12w?., Dec. 35° N. 

Lepus. — A southern constellation, situated 
R. A. 5h. 20?n., Dec. 18° S. 

Lesuth. — A star of the third magnitude in 
the constellation Scorpio, known also as 
X Scorpii. 

Leucothea. — The thirty-fifth asteroid; it 
was discovered by Luther, April 19, 
1855. 

Level. — An instrument which indicates the 
line parallel to the horizon, by means of 
a bubble of air enclosed with some fluid 
in a glass tube of an intermediate length, 
and hermetically sealed. This is called 
the air-level or spirit-level. There are 
various kinds of levels, as the water-level, 
pendulum-level, &c. A surface is said 
to be level when it is concentric with 
the surface of the sea, or with the sur- 
face the ocean would have were the globe 
entirely covered with water. 

Level Error. — The deviation of the axis of 
an instrument from the horizontal posi- 
tion. 

Libra. — One of the zodiacal constellations, 
the middle of which is R. A. 15A. 4m., 
Dec. 8° S. 

Libration in Latitude. — The axis of the 
Moon is not quite perpendicular to her 
orbit, which enables us to see sometimes 
a little naore of her polar regions than at 
others. This variation is called libration 
in latitude. 

Libration in Longitude. — The motion of 
the Moon round the Earth is not uni- 
form ; hence we see occasionally a little 
farther round each limb sometimes than 
at others, which is called the Moon's libra- 
tion in longitude. 

Libration, Parallactic— That variation 
which takes place in the visible portion 
of the Moon's surface, caused by a change 
of place in the observer's point of view, 
produced by the Earth's rotation. 

Light. — That principle by whose agency 
we derive our sensations of external ob- 
jects through the sense of sight. 



Limb. — The border or edge of the Sun or 
Moon, as the upper limb or edge, the 
lower limb, &c. It also signifies the gra- 
duated edge of a quadrant or other mathe- 
matical instrument. 

Limit. — The limit of a planet is its greatest 
heliocentric latitude. Also, the greatest 
distance at which a moon can be from her 
nodes in order that an eclipse may occur. 
See Ecliptic Limits. 

Limiting Parallels. — Those parallels of 
latitude beyond which occultations of 
fixed stars by the Moon cannot occur. 

Line of Apsides or Apses. — A straight line 
joining the apses of a planet. 

Line of CoUimation. — See Collimatiox. 

Line of Incidence. — In catoptrics, or the 
science explaining the laws of light re- 
flected from mirrors, speculse, &g., the line 
of incidence is the right line in which 
light is propagated from a radiant point 
to a point on the surface of the mirror. 
This line is also called an incident ray. 

Line of the Nodes.— The imaginary line 
joining the nodes of a planet or satellite. 

Longitude of Ascending Node. — The 
longitude of the Moon or a planet when 
in the plane of the ecliptic, moving north- 
ward. 

Longitude, Celestial. — The longitude of a 
star or planet is its distance reckoned on 
the ecliptic eastward, from the point Aries, 
quite round the globe. 

Longitude of Epoch. — See Epoch. 

Longitude, Geocentric. — The longitude of 
a planet as seen from the Earth. 

Longitude, Heliocentric. — The inclination 
to the plane of the ecliptic of the line 
drawn between the centre of the Sun and 
that of the planet. 

Longitude, Mean. — The mean or circulai 
motion of a body estimated from the 
vernal equinox. 

Longitude of Perihelion. — The longitude 
of the perihelion is the same as the longi- 
tude of a planet when at its least dis- 
tance from the Sun. 

Longitude of a Star or Planet.— Its dis 
tance reckoned on the ecliptic, from the 
point Aries, eastward round the celestial 
globe. 

Longitude Stars. — Those fixed stars which 



444 



BOUVIER S FAMILIAR ASTRONOMY. 



have been selected for the purpose of I 
finding the longitude by lunar observa- 
tions, as a Arietis, Aldeharan, <fec. 

Longitude of the Sun. — The same as the 
Sun's place in the ecliptic — that is, the 
number of degrees and minutes from the 
point Aries. 

Longitude, Terrestrial. — The distance of 
any place from any fixed meridian. 
Longitude on the Earth may be either 
east or west, as the place may be east or 
west from the fixed meridian, and is 
reckoned on the equator half round the 
globe, or 180°. 

Longitude, True. — The elliptical motion of 
a body reckoned from the vernal equinox. 

Lower Apsis. — See Apsis. 

Lucida. — The principal star in a constella- 
tion. 

Luculi. — The same ^sfaculce, which see. 

Luminary. — That which diffuses light; a 
luminous body. 

Lunar Cycle. — A period of time consisting 
of 19 years. 

Lunar Distance. — The angular distance of 
the centre of a celestial object from the 
centre of the Moon. 

Lunar Mountains. — Those protuberances 
on the Moon's surface which are visible 
by means of a telescope. 

Lunar Observation. — A method of ascer- 
taining the longitude at sea by the 
Moon's motions, particularly by her ob- 
served distances from the Sun and stars. 

Lunarians. — The inhabitants (if any) of 
the Moon. 

Lunation. — The period or time between 
one new moon and another; it is also 
called a synodical month. 

Luni-solar Precession in Longitude. — 
The motions due to the action of the Sun 
and Moon when estimated on the ecliptic. 

Lupus. — A southern constellation, situated 
in R. A. Uh. 20m., Dec. 45° S. 

Lutetia. — The twenty-first asteroid ; it was 
discovered by Goldschmidt, November 
15, 1852. 

Lynx. — A northern constellation, the mid- 
dle of which is R. A. 1h. 2im., Dec. 50° N. 

Lyra. — A northern constellation, the mid- 
dle of which is in R. A. ISh. iAm., Dec. 
38° N. 



M. — In the astronomical tables M is used 
for meridional or southern ; also for mid- 
day. 

MacMna Pneumatica or Antlia Pneuma- 
tica. — A southern constellation, the mid- 
dle of which is R. A. lOA. Om., Dec. 35° S. 

Maculae. — Dark solar spots, surrounded by 
a band or border less completely black, 
called a penumbra. 

Magnetic Amplitude. — An arc of the hori- 
zon intercepted between the Sun in his 
rising or setting and the east and west 
points of the compass. 

Magnetic Meridian. — See Meridian. 

Magnetic North. — The direction of the 
magnetic meridian which passes through 
the magnetic poles and the place of the 
observer. 

Magnifying Power. — The amount of appa- 
rent enlargement of an object by means 
of a lens. 

Magnitude. — That which is made up of 
parts locally extended, as a line, surface, 
or solid. The stars are said to be of the 
first, second, third, &c. magnitude, ac- 
cording to their brilliancy, without refer- 
ence to their real size, which is unknown. 

Magnitude, Apparent. — The visual angle 
formed by rays drawn from the extre- 
mities of a body to the centre of the eye. 

Magnitude of an Eclipse. — The proportion 
which the eclipsed portion of the disc of 
the Sun or Moon bears to the diameter. 
It is sometimes expressed in digits, but 
more frequently as a decimal, the diame- 
ter being taken as unity. 

Maia. — A name given to one of the Pleiades. 

Major Axis.— See Axis. 

Malus. — One of the divisions of the con- 
stellation Argo Navis, as made by some 
astronomers. It signifies the mast. 

Manilius. — The name of a lunar mountain. 

Map. — A representation on a plane surface 
of a portion of the heavens, or of the 
surface of the Earth ; the former is called 
a celestial, the 'latter a terrestrial, map. 

March. — The third month in the year, ac- 
cording to our calendar ; but in the old 
Roman year it stood first, in honor of 
Mars. 

Mare Crisium. — A name designating a flat 
portion of the Moon's sphere. It was 



ASTRONOMICAL DICTIONARY, 



445 



called mare because it was Ibrmerly 
thought to be a sea. 

Mare Humorum. — A spot on the surface 
of the Moon designated by the name of 
a sea, 

Marfak. — A name given by the Arabians 
to the stars n and B in the constellation 
Cassiopeia. 

Markab. — A bright star in the constella- 
tion Pegasus, forming the south-eastern 
corner of the Square. It is also known 
as a Pegasi. 

Markeb. — A small star in the constellation 
Argo Navis, marked k Argo Navis. 

Mars. — The fourth planet in order from 
the Sun. 

Marsik. — A binary star in Ophiuchus, also 
known as X Ophiuchi. 

Mass. — The entire amount of matter con- 
tained in a body ; the weight or attract- 
ive power of a planet expressed in refer- 
ence to that of the Sun. 

Massilia. — The twentieth asteroid in order 
of discovery. It was seen by Gasparis, at 
Naples, on the 19th of September, 1852; 
and by Chacornac, at Marseilles, on the 
following evening, the 20th of September. 

Masym. — A star in the constellation Her- 
cules, known as X Herculis. 

Matter. — That which is the object of our 
senses. 

Maximum. — The greatest quantity or de- 
gree attainable in any given case. 

May. — The fifth month in our year, but 
the third month according to the ancient 
Roman reckoning. It was called Mains 
by Romulus, in respect to the senators 
and nobles of Rome, who were called 
Majores. Some say it was thus called 
from Maia, the mother of Mercury, to 
whom they offered sacrifice on the first 
day of this month. 

Mean. — A middle state between two ex- 
tremes. 

Mean Distance of a Planet from the Sun. 
— An arithmetical mean between the 
planet's greatest and least distances, 
and this is equal to the semi-transverse 
axis of the elliptical orbit in which it 
moves. 

Mean Equinox. — The estimated place of 
the vernal equinox without nutation. 



Mean Equinox, Eeduced Place of.— The in- 
tersection of a declination circle through 
the mean equinox with the equinoctial. 

Mean Motion. — The mean angular velocity 
of a body. The rate at which a body 
moving in an elliptic orbit would pro- 
ceed, had it to describe the whole cir- 
cumference at an equal velocity through- 
out. 

Mean Noon. — The time of the mean Sun 
being on the meridian of a place. 

Mean Obliquity. — The inclination of the 
equinoctial to the ecliptic without nuta- 
tion. 

Mean Sidereal Time. — Time measured by 
the hour-angle of the mean equinox. 

Mean Solar Time. — That time which is in- 
dicated by our clocks and watches, which 
is regulated by the motion of an imagi- 
nary Sun supposed to move uniformly in 
the equator. See Mean Sun. 

Mean Sun. — Astronomers assume the revo- 
lution of a sun in a circular orbit in the 
plane of the equator, having the mean 
diurnal motion in right ascension of the 
true Sun. The time which elapses be- 
tween two successive transits of the mean 
Sun constitutes a mean solar day, which 
is the measure of time in common use. 

Mean Time. — That time which is measured 
by any equable motion, as that of a clock. 

Medium. — The space in which a ray of 
light moves ; it may be either pure space, 
air, water, glass, diamond, or any other 
transparent substance through which 
rays of light can pass in straight lines. 

Medium Coeli. — Mid-heaven. 

Medium, Resisting. — A thin ethereal mat- 
ter, supposed to perv^ade all space, and 
which resists the progressive motions of 
the periodical comets, so that their velo- 
city is diminished, and consequently their 
orbits contracted, at every revolution. 

Megallanic Clouds. — See Nubecula. 

Megaloscope. — A species of telescope for 
viewing objects which are very near. 

Megrez. — A star in the constellation Ursa 
Major, called also 6 Ursa3 Majoris. 

Melpomene. — The eighteenth asteroid; it 
was discovered by Hind, June 24, 1852. 

MencMb. — A star in the constellation Per- 
seus, called also ^ Persei. 



BOrVIER S FAMILIAR ASTRONOMY. 



Menkalinan. — A star in the constellation 
Auriga, called also ,5 Aurigae. 

Menkar. — A star in the constellation Cetus, 
also called a Ceti. 

Menstrual Argument of Latitude. — See 
Argumext. 

Merak. — A bright star in the constellation 
Ursa Major, known as /? UrsEe Majoris. 
This star is one of the Pointers. 

Mercury. — The planet nearest to the Sun, 
around which it is carried with a very 
rapid motion. Hence the Greeks called 
this planet Mercury, after the name of 
the nimble messenger of the gods. 

Meridian. — A great circle of the sphere 
passing through the zenith and the poles. 
It is so called from the Latin meridies, 
midday, because when the Sun is on the 
meridian it is noon to all places situated 
under it. 

Meridian Altitude. — An arc of the meri- 
dian intercepted between the horizon and 
the centre of an object on the meridian. 

Meridian Line. — The intersection of the 
plane of the meridian with the sensible 
horizon. 

Meridian, Magnetic. — The intersection of 
the surface of the Earth with a vertical 
plane passing through the magnetic poles 
and the given place. 

Meridian Mark. — A mark on a pillar, or 
some fixed object in the line of the meri- 
dian, and situated at a proper distance 
from the observatory. 

Meridian Passage. — The transit of a pla- 
net over the meridian of any place. 

Meridian, Plane of. — An imaginary sur- 
face supposed to pass through the meri- 
dian and to be extended to the celestial 
sphere. 

Meridian, Kational. — The intersection of 
the rational horizon with the meridian 
plane of the place. 

Meridian, Terrestrial. — A line passing 
through the poles of the Earth, all the 
points of which line have contemporane- 
ously the same noon. 

Meridian Transit. — The passage of a hea- 
venly body across the meridian. 

Merope. — One of the Pleiades. 

Mesartim. — A star in the constellation 
Aries, known also as y Arietis. 



Meteor. — A fiery or luminous body occa- 
sionally seen moving rapidly through the 
atmosphere, sometimes leaving behind it 
a luminous train of light, which continues 
for several seconds. Meteors are vul- 
garly called/a/^i 7!5r stars, or shooting stars. 

Meteoric Stones or Meteorites. — Stones or 
semi-metallic substances which fall from 
the upper regions of the atmosphere. 

Metis. — An asteroid discovered by Gra- 
ham, April 25, 1848, and is the ninth in 
order of discovery. 

Metonic Cycle. — A cycle of 19 years, so 
called from Meton, the Athenian, by 
whom it was discovered. See Cycle of 
THE Moon. 

Miaplacidus. — A star of the first magni- 
tude in the constellation Argo Kavis, 
called ,(3 Argo Xavis. 

Micrometer, {y-iKpog, small, and iitrpov, a 
measure.) — An astronomical instrument 
to measure small angular distances. 

Micrometer, Position. — A micrometer hav- 
ing a single thread or wire, which is car- 
ried round, by a smooth revolving mo- 
tion, in the common focus of the object 
and eye-glasses, and in a plane perpen- 
dicular to the axis of the telescope. It 
is used to determine the situation of two 
fixed stars with regard to each other, if 
they both appear in the field of the tele- 
scope at the same time. 

Microscopium. — A southern constellation, 
the middle of which is R. A. 20/i. 40;«., 
Dec. 38° S. 

Midday, Noon. — That point of time when 
the Sun is on the meridian of a given place. 

Mid-heaven or Medium Coeli. — That point 
of the ecliptic which culminates, or is on 
the meridian, at any time. See also 

XOXAGESIMAL. 

Midnight. — That point of time opposite to 
noon. At midnight the Sun is on the 
meridian 180° from us. 

Milky Way.— See Galaxt. 

Mimas. — The innermost or first satellite 
of Saturn ; discovered by Sir WUliam 
Herschel, September 17, 1789. 

Minimum. — The least quantity. 

Minor Axis. — The shorter or conjugate dia- 
meter of the ellipse, and perpendicular to 
the longer or transverse diameter. 



ASTRONOMICAL DICTIONARY. 



447 



Mintaka. — The name of a star of the se- 
cond magnitude in Orion's belt. It is 
also known as 6 Orionis. 

Minute. — The sixtieth part of an hour; 
and also the sixtieth part of a degree of 
angular measurement. 

Mira, {mirahiUs, wonderful.) — A variable 
star in the neck of Cetus, also called o 
Ceti, or sometimes CoUo Ceti. It passes 
through all its changes in 334 days, dur- 
ing which time it exhibits some remark- 
able irregularities. 

Mirach. — A star in the constellation An- 
dromeda, known as /? Andromedae. 

Mirfak. — An Arabic name for a star in the 
constellation Perseus, known also as o 
Persei. 

Mirror. — The surface of an opaque body 
polished and fitted to reflect the rays 
of light that fall upon itj a looking- 
glass. 

Mirzaxn. — A star in the constellation Canis 
Major, known as /? Canis Majoris. 

Mizar. — A bright star in the constellation 
Ursa Major, also called ^ Ursae Majoris. 

Mock Sun. — See Parhelion. 

Momentum. — The onward impulse or force 
of motion of a body. 

Monoceros. — A southern constellation, the 
middle of which is R. A. 7A. 20»i., Dec. 
0° S. 

Mons Menalus. — A northern constellation, 
the middle of which is R. A. 14A. 40hi., 
Dec. 15° N. 

Mens Mensse. — A southern constellation, 
the middle of which is R. A. bh. 20?n., 
Dec. 75° S. 

Month, (Saxon, monath, and Teutonic, 
3Iond, the Moon.) — The twelfth part of 
the year, and is so called from the Moon, 
by whose motions it is regulated, being 
properly the time which the Moon moves 
through the zodiac. 

Month, Anomalistic. — The time the Moon 
requires to move from perigee and return 
to that point again. 

Month, Nodical.— The time occupied by 
the Moon in completing a revolution from 
one node to the same node again. 

Month, Solar.— The time in which the Sun 
appears to move through one sign of the 
zodiac. 



Month, Synodic. — Called also a lunation. 
See Lunation. 

Moon. — The satellite belonging to our 
Earth. 

Moon-culminating Stars. — Those stars 
which, being near the Moon's parallel of 
declination, and not differing much from 
her in right ascension, are proper to 
be observed with the Moon, in order 
to determine the differences of meri- 
dians. 

Moon, Full. — That appearance of the Moon 
when she is in opposition to the Sun, or 
when her full orb shines on us. 

Moon, New. — That phase of the Moon 
which happens when she is in conjunc- 
tion with the Sun. 

Motion. — A continued and successive 
change of place. 

Motion, Absolute. — An absolute change 
of place considered independently of any 
other body. 

Motion, Accelerated or Eetarded. — That 
motion, the velocity of which is continu- 
ally increased or diminished. 

Motion, Angular. — That motion by which 
the angular position of any body is 
changed. 

Motion, Compound. — That motion pro- 
duced by two or more powers acting in 
different directions. 

Motion, Direct. — A planet's motion is said 
to be direct when it moves in its orbit 
from west to east, or according to the 
order of the signs. 

Motion, Diurnal. — The angular motion de- 
scribed by a heavenly body in twenty- 
four hours. 

Motion, Equable or Uniform. — That mo- 
tion by which a body proceeds with ex- 
actly the same velocity, always passing 
over equal spaces in equal times. 

Motion, Horary. — That motion which takes 
place during an hour. 

Motion, Proper.— Change of place of the 
fixed stars is called their proper motion. 
This term is also applied to the motion 
of a planet from west to east, or accord- 
ing to the order of the signs. 

Motion, Relative. — The change of the rela- 
tive place of a moving body with regard 
to another body also in motion. 



448 



BOUVIER S FAMILIAR ASTRONOMY. 



Motion, Retrograde. — When the real or 
apparent motion of a heavenly body is 
from east to west, it is said to be retro- 
grade. 

Motion, Simple. — That motion which is 
produced by some one power or force 
only. 

Motion, Sidereal. — The motion of the 
Earth or a heavenly body with respect 
to the fixed stars. 

Motion, True. — That motion which a hea- 
venly body actually performs. 

Motion, Tropical. — The motion of a body 
with respect to the equinox or tropic. 

Mufrid. — A star of the third magnitude in 
the constellation Bootes, also known as 
rj Bootis. 

Muliphien. — A name applied to a variable 
star in the constellation Canis Major. 
The stars a and /? Columbae are also 
called Muliphien. 

Multiple Stars. — Four or more stars united 
together, forming one system, and revolv- 
ing round a common centre of gravity. 

Mural Arch, {muralis, a wall.) — An instru- 
ment or quadrant fixed against a wall or 
pillar, having its face in the plane of the 
meridian, for the purpose of making ob- 
servations. 

Mural Circle. — A graduated circle, the axis 
of which is placed due east and west, and 
firmly fixed in a horizontal position in 
the face of a stone pier or wall, and 
erected in the plane of the meridian. It 
is used to determine the declinations of 
the heavenly bodies. 

Mural Quadrant. — An astronomical quad- 
rant fixed in the plane of the meridian 
against a substantial wall. It is hence 
denominated a mural quadrant. 

Musca Australis. — A southern constella- 
tion, situated R. A. \2h. 20m., Dec. 70° S. 

Musca Borealis. — A northern constellation, 
the middle of which is R. A. 2h. 4:0m., 
Dec. 28° N. 

Kadir. — That point of the heavens directly 
under the feet of an observer; that point 
opposite to the zenith. 

Nadir, Apparent. — That point in which a 
plumb-line extended intersects the celes- 
tial sphere under our feet. 



Nadir of the Sun. — The axis of the conical 
shadow projected by the Earth. 

Naos. — A name given to the star ^ in the 
constellation Argo Navis. 

Naschirah. — Another name for 6 Capricorni, 
a star in the constellation Capricornus. 

Natural Day. — The time in which the 
Earth rotates on its axis. 

Nautical Almanac or Astronomical Ephe- 
meris. — An almanac containing the lati- 
tudes, longitudes, ascensions, declina- 
tions, &c. of the heavenly bodies. It is 
intended for the use of mariners, as well 
as astronomers; and the calculations are 
for two or more years in advance. 

Neap Tides. — Those tides which occur soon 
after the Moon's quadratures, or first and 
last quarters. 

Nebula. — A collection of stars so closely 
congregated as to require large telescopes 
to separate them. — Hind. 

Nebulae, Double. — Those nebulae which ap- 
pear near to each other when viewed 
through the telescope. 

Nebulae, Irregular. — Those nebulae which 
appear to be destitute of any symmetry 
of form. 

Nebulae, Irresolvable. — Those nebulae 
which are at too great a distance for our 
telescopes to separate, so as to discern 
their component stars. Through the best 
instruments they appear like spots of 
cloudy light. 

Nebulae, Planetary. — Those nebulae hav- 
ing discs resembling planets, being some- 
times quite round, at others oval, with a 
hazy edge or outline. They are very 
rare, most of those yet discovered being 
in the southern hemisphere. 

Nebulae, Spiral. — Nebulae which appear to 
consist of a spiral arrangement of stars 
diverging from a centre. 

Nebular Hypothesis. — A theory advanced 
by La Place, which assumed that the ne- 
bulous matter which surrounds some stars 
was diS"used throughout the universe, and 
that by its attraction and condensation 
this nebulous matter gradually formed 
clusters of stars. More recent disco- 
veries, however, tend to show the fallacy 
of this theory. — Hind. 

Nebulous Stars. — Those stars which are 



ASTRONOMICAL DICTIONARY. 



449 



surrounded by a circular disc or atmo- 
sphere of faint light. 

Nebulous Double Stars.— Double stars sur- 
rounded by a faint atmosphere of light. 

Nekkar. — A bright star in the constella- 
tion Bootes, and known as /3 Bootis. 

Neptune. — The most distant known pla- 
net in our system. 

Newton. — The name of an elevated peak 
on the Moon. 

Nihal. — The name of a star in the constel- 
lation Lepus, commonly known as /? Le- 
poris. 

Nocturnal Arc. — The arc of the heavens 
described by a celestial body in the night. 

Node, Ascending.— See Ascending Node. 

Node, Descending. — See Descending 
Node. 

Node, longitude of.— The distance of the 
ascending node from the first point of 
Aries. 

Nodes, (nodus, a knot.) — The two opposite 
points in a planet's orbit where it inter- 
sects the ecliptic. 

Nodes, Line of. — See Line of Nodes. 

Nodus Primus. — A star situated in Draco, 
known also as ^ Draconis. 

Nodus Secundus. — A bright star in the con- 
stellation Draco, also called 6 Draconis. 

Nodical Revolution. — The passage of the 
Moon from one node to the same node 
again. 

Nonagesimal, called also Mid-heaven. — 
It is the highest point, or 90th degree of 
the ecliptic, reckoned from its intersec- 
tion with the horizon. 

Nonius. — A name given erroneously for 
Vernier. 

Norma. — A southern constellation, the mid- 
dle of which is R. A. 16h. 8m., Dec. 50° S. 

Normal. — In mathematics, another name 
for perpendicular. 

North. — One of the four cardinal points. 
That point of the horizon opposite to the 
Sun when on the meridian at noon. 

North Polar Distance.— The distance of a 
celestial body from the north pole of the 
heavens. 
North Pole. — A point in the northern he- 
misphei-e of the heavens, 90° every way 
distant from the equinoctial. 
North Following.— That portion of the 



quadrant situated between the north and 
east points. 

North Preceding. — A term used to indi- 
cate a point of the quadrant between the 
north and west points. 

North Star.— See Cynosttra. 

Northern Constellations. — See Constel- 
lations. 

Northern Light. — See Aurora Borealis. 

Northern Signs. — Those six signs of the 
zodiac which are north of the equinoc- 
tial — viz. : Aries, Taurus, Gemini, Can- 
cer, Leo, and Virgo. 

Nova. — New. — A term used by Sir J. Her- 
sehel in his catalogue, signifying stars 
or nebula which have never before been 
discovered. 

November. — The eleventh month in our 
year, but the ninth in the Roman year, 
whence its name, from Novem, nine. 
About the 21st day of this month the 
Sun enters Sagittarius. 

Nubeculge or Magellanic Clouds. — Two 
large nebulae near the southern pole of 
the equator; they are called nubecula 
major and minor. 

Nucleus. — The solid part of any thing. 
The central part of the body or head of 
a comet. 

Nutation. — A libratory motion of the 
Earth's axis, discovered by Dr. Bradley, 
by which its inclination to the plane of 
the ecliptic is continually varying by a 
small amount. 

Nutation of Obliquity. — The difference be- 
tween the mean and apparent obliquity 
of the ecliptic. 

Nycthemeron. — Natural day. See Day. 

Oberon. — A name given by Sir John Her- 

schel to the third satellite of Uranus. 
Obfuscate. — To darken. During a solar 

eclipse the Sun is said to obfuscate; that 

is, to become obscured. 
Object-glass. — The glass of a telescope or 

microscope which is placed at the end of 

the tube which is next or towards the 

object to be viewed. 
Oblate.- -Flattened. 
Oblate Spheroid. — See Spheroid. 
Oblique. — Not perpendicular; inclined. An 

angle more or less than a rii^ht ansjle. 



29 



450 



BOUVIER S FAMILIAR ASTRONOMY. 



Oblique Ascension. — That point of the 
equinoctial which rises with the centre 
of the Sun or a star in an oblique sphere. 

Oblique Descension. — That point of the 
equinoctial which sets with the centre of 
the Sun or a star in an oblique sphere. 

Oblique Sphere. — That position of the 
Earth when the rational horizon cuts the 
equator obliquely ; hence it derives its 
name. All the inhabitants of the Earth, 
except those who live at the poles or on 
the equator, have this position of the 
sphere ; their days and nights are of 
unequal lengths, the parallels of lati- 
tude being divided into unequal parts 
by the rational horizon. 

Obliquity of tlie Ecliptic— The angle 
which the ecliptic makes with the equa- 
tor. 

Observatory. — A building designed for the 
reception of astronomical instruments, 
and for making observations on the hea- 
venly bodies. 

Observation. — The act of observing with 
an instrument some celestial phenomena, 
such as the altitude of the Sun, Moon, 
stars, <fec. By this term seamen mean 
only taking the meridian altitudes in 
order to find the latitude. 

Obtuse Angle. — One that is greater than a 
right angle, or that consists of more 
than 90°. 

Occidental. — Westward. A planet is said 
to be occidental when it sets after the 
Sun. 

Occultation. — The eclipse of a planet or 
fixed star by another planet or the Moon. 

Occultation, Circle of Perpetual. — A cir- 
cle at a certain distance from the poles 
which contains all those stars never seen 
in our hemisphere. The situation of this 
circle of course depends on the latitude 
of the observer. 

Occulted. — A body is said to be occulted 
when it is hidden or eclipsed by another 
body coming between it and the observer. 
This term is chiefly applied to the eclipses 
of planets or stars by the Moon. 

Octaeteris, (okto), eight, and tros, year.) — A 
cycle or term of eight years, at the end 
of which three lunar months were added. 
This cycle was in use until Meton, the 



Athenian, reformed the calendar by dis- 
covering the golden number, or cycle of 
nineteen years. 

Octans Hadleianus. — A southern constel- 
lation, situated R. A. 21^., Dec. 85° S. 

Octant or Octile. — An aspect or position 
of two planets, when their places are dis- 
tant by the eighth part of a circle, or 45°. 

October, [oktw, eight.)— This is now the 
tenth month of the year, but was the 
eighth in the calendar of Romulus. About 
the 22d of this month the Sun enters 
Scorpio. 

Olympiads. — Periods of time consisting of 
four Grecian years. The name is de- 
rived from Olympia, where public games 
were celebrated every fourth year. 

Opaque Body. — One that is impervious to 
the raj-s of light; that cannot transmit 
light. An opaque body will cast a sha- 
dow, and receive one that is cast by some 
other body. 

Ophiuchus, sometimes called Serpentarius. 
— The middle of this constellation is R. 
A. irh. Sm., Dec. 0° 0'. 

Opposition. — A planet is said to be in op- 
position when its longitude differs from 
that of the Sun by 180°. 

Optical Double Star. — See Double Star. 

Optics. — That branch of natural philosophy 
which treats of the nature and properties 
of light. 

Orbit. — The path of a planet or comet 
round the Sun, or of a satellite round its 
primary. The path of any heavenly 
body, or system of bodies, round its cen- 
tre of gravity. 

Orbitual. — Relating to an orbit. 

Orbus Magnus. — Another name for the 
orbit of the Earth. 

Order. — A heavenly body is said to move 
in the order of the signs when it pro- 
ceeds from Aries to Taurus, thence to 
Gemini, &c. See Consequextia. 

Orient — The east, or eastern point of the 
horizon. 

Oriental. — Easterly. When a planet rises 
before the Sun it is said to be oriental. 

Orion, (called by the Arabians Almahha-'- 
rah, the brave warrior.) — A splendid 
southern constellation, the middle of 
which is in R. A. 5A. 20»i., Dec. 0°. 



ASTRONOMICAL DICTIONARY. 



451 



Orrery. — An astronomical machine for ex- 
hibiting the various motions of the Sun 
and planets ; it was so called in honor 
of the Earl of Orrery, by whom the inven- 
tion was first patronized : it was after- 
wards called a planetarium. 

Orthogonal, (opdog, right, and yovia, an an- 
gle.) — A term given to any figure which 
has one or more right angles. 

Ortive Amplitude or Eastern Amplitude, 
— An arc of the horizon intercepted be- 
tween the point where a star rises and 
the east point of the horizon. 

Oscillation. — A motion to and fro like the 
pendulum of a clock. 

Oscillation, Axis of. — A line parallel to the 
horizon, and supposed to pass through 
the point about which a pendulum oscil- 
lates ; it is perpendicular to the plane in 
which the oscillation is made. 

Falilicium, — Another name for Aldeharan, 
a bright star in the constellation Taurus ; 
the same name was given by Pliny to 
the Hyades, a group of stars in the same 
constellation. 

Pallas. — The second asteroid in the order 
of discovery. It was first observed by 
Dr. Olbers, March 28, 1802. 

Parabola. — A figure arising from the sec- 
tion of a cone when cut by a plane pa- 
rallel to one of its sides. 

Paracentric, {napa, beyond, and Ktvrpov, cen- 
tre.) — Deviating from the centre. A curve 
having the property that, when its plane 
is placed vertically, a heavy body de- 
scending along it, urged by the force of 
gravity, will approach to, or recede from, 
a fixed point or centre, by equal distances 
in equal times. — Davies and Peck, Math. 
Diet. 

Paracentric Motion. — A term indicating 
the quantity which a revolving body ap- 
proaches to, or recedes from, its centre of 
motion or attraction. 

Parallactic Angle. — See Parallax. 

Parallactic Inequality. — An inequality in 
the Moon's motion arising from the sen- 
sible difference in the Sun's disturbing 
force, when the Moon is in that portion 
of her orbit nearest to the Sun, or when 
iu the opposite point of her path. 



Parallactic Libration. — See Libration. 

Parallactic Motion. — The motion of a body 
when the space described by it subtends 
or is seen under a sensible angle. 

Parallax. — The angle under which an ob- 
ject is seen from the Earth. As our dis- 
tance is increased, the parallax is dimi- 
nished. The difference between the ap- 
parent position of the Sun, Moon, or 
planets, when viewed from the surface of 
the Earth and from its centre, is called 
the parallax. 

Parallax, Annual.— The angle which the 
diameter of the Earth's orbit would sub- 
tend if viewed from one of the heavenly 
bodies. The change of place observable 
in a few of the fixed stars, which is due 
to the motion of the Earth round the 
Sun, is called the annual parallax. 

Parallax, Diurnal. — The change of place 
in a heavenly body arising from the mo- 
tion of the Earth upon her axis, is some- 
times called the diurnal parallax. 

Parallax, Equatorial Horizontal. — The 
greatest angle subtended by the Earth's 
equatorial semi-diameter as seen from 
the Sun or Moon. 

Parallax, Horizontal. — The parallax of a 
celestial body when in the horizon. As 
the body rises above the horizon the pa- 
rallax diminishes until it arrives at the 
zenith, where the parallax vanishes. 

Parallel Lines, {napa, against, and a\\ri\wv, 
one another.) — Lines having the same 
direction ; though two parallel lines were 
infinitely extended, they would always be 
equidistant from each other in all their 
parts. 

Parallel Rays. — Those rays which are 
always at an equal distance, in respect to 
each other, from the visual object to the 
eye, from which the object is supposed 
to be infinitely distant. 

Parallelism of the Earth's Axis.— The 
parallel position which the Earth always 
preserves in regard to itself in its orbit 
round the Sun ; so that if a line be drawn 
parallel to its axis, while in any one part 
of its orbit, the axis in all other parts 
will be parallel to the same line. 

Parallelogram, (TapaXXrjXo», parallel, and 
ypanfia, a diagram.) — A quadrilateral 



452 



BOUVIER S FAMILIAR ASTRONOMY. 



figure whose opposite sides are parallel 
and equal taken in pairs, and also its op- 
posite angles. 

Parallelogram of Forces.— A term used to 
denote the composition of forces, or the 
finding a single force that shall be equi- 
valent to two or more given forces act- 
ing in given directions. 

Parallels of Altitude.— See Almacaxtars. 

Parallels of Declination. — Lesser circles 
of the celestial sphere parallel to the 
equinoctial. 

Parallels of Latitude.— In astronomy, 
lesser circles parallel to the ecliptic ; but 
in geography, they are lesser circles pa- 
rallel to the equator. 

Parallel Sphere. — That situation of the 
sphere where the equator coincides with 
the horizon, and the poles with the ze- 
nith and nadir. This is the position of 
the sphere to the inhabitants, if any, at 
the poles. 

Paraselene. — A meteor in a cloud resem- 
bling the Moon ; a moch vioon. 

Parhelion or Mock Sun. — {-apa, near, and 
TjXioj, the Sun.) — A phenomenon usually 
accompanying the coronae or luminous 
circles which sometimes surround the 
Sun. The mock suns are often very 
bright, being formed by the reflection 
and refraction of the Sun's rays. 

Parthenope. — An asteroid, discovered May 
13, 1850, by Gasparis. It is the eleventh 
in order of discovery. 

Partial Eclipse. — An eclipse of the Sun or 
Moon in which only a part of the disc is 
obscured. 

Path. — A term applied to the curve made by 
any heavenly body as it moves through 
space. 

Pavo. — A southern constellation, in R. A. 
19^. 40m., Dec. 68° S. 

Pegasus. — A northern constellation, the 
middle of which is R. A. 23h. Qm., Dec. 
5°K 

Pencil of Bays. — A system of rays diverg- 
ing from a point. A double cone or py- 
ramid of rays diverging from some lumi- 
nous point, and which, after falling on, 
and passing through, a lens, converges 
again on entering the eye. 

Pendulum. — Any heavy body so suspended 



that it may vibrate about some fixed point 
by the force of gravity. The vibrations 
of a pendulum are called its oacillations ; 
the time of each oscillation being the 
time required for the pendulum to pass 
from the highest point on one side to the 
highest point on the other side. 

Penumbra. — In an eclipse, a faint or par- 
tial shade observed between the perfect 
shadow and the full light. Also, the 
border or margin surrounding the dark 
spots on the Sun. 

Periaster,(-£:p(, around, and a;Tpov, a star.) — 
That point in the orbit of a double or 
compound star nearest to its primary. 

Perigean Tides. — Those spring-tides which 
occur soon after the Moon passes her 
perigee. 

Perigee, (from r-cpi, about, and yj?, the Earth.) 
— That point of the orbit of the Moon 
which is the nearest to the Earth. 

Perihelion, (-s-pt, around, andJ/Xio,-, the Sun.) 
— That point in a planet's orbit nearest 
to the Sun. 

Perihelion Distance. — The least distance 
of a planet or comet from the Sun. 

Perihelion,Longitude of.— See Longitude. 

Perihelion Passage. — The moment of time 
when a planet or comet is in that point 
of its orbit nearest to the Sun. 

Perimeter, {~^pi, around, and psrpov, a mea- 
sure.) — The sum of the boundary lines 
of any figure. In any circular figure, 
the terms circumference and periphery 
are used. 

Period. — The time which a star, planet, or 
comet requires to perform one revolu- 
tion round its centre of gravity; also, a 
series of years by which time is mea- 
sured by difi"erent nations ; as, the Julian, 
the Calippic, the Metonic, the Chaldaic 
period. 

Period of the Eclipses. — A period of time 
in which the Sun and Moon will be found 
nearly in the same position with respect 
to the place of the Moon's node. This 
period consists of 223 lunations, or 6585 
days. 

Periodic. — A term applied to the motions 
or courses of the heavenly bodies per- 
formed within any given space of time. 

Periodic Inequalities. — Those disturb- 



ASTRONOMICAL DICTIONARY. 



453 



anees in the motions of the planets 
which are caused by their reciprocal at- 
traction, depending upon their positions 
with regard to each other, and which 
are accomplished in a definite period. 

Periodical Month. — The time required by 
the Moon to perform one revolution 
round the Earth. 

Feriodio Time. — The interval between two 
consecutive passages of a planet through 
the same node. 

Periodic Variation. — A regular or suc- 
cessive variation in the motions of the 
heavenly bodies. 

Perioeci, (^rEpi, about, around, and oiko^, a 
house.) — Those inhabitants of the Earth 
who live in the same latitude, but oppo- 
site longitudes. 

Periphery, (wpj, about, and depo, to carry.) 
— The circumference of any curve, as 
the circle, ellipse, parabola, &c. 

Periscii, (^^P', about, and ama, a shadow.) — 
The inhabitants of the frigid zones are 
so called because their shadows turn 
round to all the points of the compass in 
one day ; for as the Sun does not set to 
them onee in twenty-four hours, their 
shadows are directed to every point of the 
compass in that time. 

Perisaturnium. — That point of the orbit 
of any of Saturn's satellites which is 
nearest to Saturn, its primary. 

Perpendicular. — At right angles with. 
One line is perpendicular to another 
when the former meets the latter so as 
to make the angles on both sides of it 
equal to each other. 

Perseus. — A northern constellation, the 
middle of which is R. A. 3A. 12m., Dec. 
48° N. 

Perturbations. — Inequalities in the mo- 
tions of the planets owing to their mu- 
tual attraction. Perturbations are of 
two kinds: periodic and secular. The 
former are accomplished in short spaces 
of time, as a few months, years, or even 
hundreds of years ; the latter require 
immense periods of time, and are there- 
fore called secular inequalities. 

Petavius. — An inequality on the surface 
of the Moon to which this name has been 
given. 



Phaet. — A name given to the star a in the 
constellation Columba. 

Phases, (<paivo}, to appear, or shine.) — 
Changes in the illuminated discs of the 
planets or the Moon, from the crescent 
to the full orb. 

Phase, Greatest. — The moment when the 
largest portion of the disc of the Sun or 
Moon is obscured during an eclipse. 

Phecda. — A star in the constellation Ursa 
Major; also termed y Ursae Majoris. 

Phenomenon. — An extraordinary appear- 
ance in the heavens or on earth ; either 
discovered by observation of the celes- 
tial bodies, or by physical experiments. 

Pherkad Major. — A name given to the 
brighter of the two stars composing the 
double star y Ursse Minoris. 

Pherkad Minor.— A name given to the 
comes of y Ursae Minoris, a star in the 
constellation Ursa Minor. 

Phocea. — An asteroid, discovered by Cha- 
cornac April 6, 1853. It is the 25th in 
the order of discovery. 

Phoenix. — A southern constellation, situ- 
ated in R, A. 0^. 4.0m., Dec. 50° 0' S. 

Photometer, (^cj;, light, and nerpov, a mea- 
sure.) — An instrument for measuring the 
illuminating powers of different sources 
of light. 

Photometry. — The science of measuring 
light. 

Phurud. — A star in the constellation Canis 
Major ; also called ^ Canis Majoris. 

Physical Astronomy. — That department 
of astronomy which treats of the motions 
of the heavenly bodies, and the laws 
which operate to produce them. 

Pico. — An inequality on the lunar surface 
to which this name is given, 

Pisces. — One of the twelve zodiacal con- 
stellations, the middle of which is R. A. 
Oh. 20m., Dec. 10° N. 

Pisces Australis. — A southern constella- 
tion, in R. A. 22h. 20m., Dec. 30° S. 

Pisces Volans. — A southern constellation. 
Its R. A. is Sh. 22m., Dec. 68° S. 

Place. — That part of space occupied by 
anybody. The place of the Sun, Moon, 
or a planet, is that point of the zodiac 
occupied by the luminary. 

Place, Apparent. — That point in which a 



454 



bouvier's familiar astronomy. 



heavenly body appears when viewed 
from the surface of the Earth ; that point 
to which the eye of an observer refers 
the object. 

Place, True. — That point in which a hea- 
venly body would appear if viewed from 
the centre of the Earth. 

Plane, (planus, even.) — A figure or sur- 
face lying evenly between its boundary 
lines. Euclid. 

Planet, {-\am(,>, to icander; i. e. literally, 
a wandering star.) — A celestial body re- 
volving round the Sun or other planet as 
a centre. 

Planets, Exterior or Superior.— Those pla- 
nets whose orbits are beyond or without 
the orbit of the Earth. 

Planets, Inferior. — Otherwise called inte- 
rior planets, which see. 

Planets, Interior. — The planets Mercury 
and Venus are called interior planets 
because their orbits are within that of 
the Earth. 

Planets, Primary. — Planets which move 
round the Sun as a centre. 

Planets, Secondary. — Planets which re- 
volve round another planet as their centre. 

Planets, Telescopic. — See Telescopic Pla- 
nets. 

Planetarium. — See Orrert. 

Planetary Nebulae. — Xebulae exhibiting 
discs of uniform brightness, but not re- 
solvable into stars. 

Planisphere. — The sphere or globe pro- 
jected on a plane surface. 

Platonic Year.— The period of time de- 
termined by the revolution of the equi- 
noxes. It is sometimes called the great 
year. 

Pleiades. — A group of stars in the con- 
stellation Taurus. 

Plumb-line, or Plummet, (plumbum, lead.) 
— A cord or string having a piece of lead 
attached at one extremity. It is used by 
astronomers for the purpose of adjusting 
an instrument to the perpendicular. 

Poetical Rising and Setting.— See Achro- 
NicAL, CosMicAL, and Heliacal, rising 
and setting of the stars. 

Point. — In astronomy, a term applied to 
certain fixed places in the heavens. Thus, 
the four grand divisions of the horizon, 



East, "West, IS'orth, and South, are called 
the cardinal points ; the zenith and nadir 
are the vertical points; the intersection 
of the equator and ecliptic are called 
the equinoctial points; the Sun's highest 
ascent above and greatest descent below 
the equinoctial are called the solstitial 
points; those points in the orbit of a 
planet which intersect the ecliptic are 
called the nodal points. 

Pointers. — This name is given to the stars 
Merah and Dubhe, in Ursa Major, be- 
cause a line drawn through them and 
extended will designate the pole star. 

Polar. — Relating to the poles. 

Polar Axis of a Telescope. — That axis of 
an equatorial instrument which is paral- 
lel to the axis of the Earth. 

Polar Circles. — Two lesser circles of the 
sphere, distant 23° 28' from each pole. 
The northern is called the Arctic, and the 
southern the Antarctic Circle. 

Polar Compression. — The flattening of the 
Earth's planet at the poles. 

Polar Diameter. — That diameter of a 
sphere which passes through the centre 
and is terminated both ways by the 
poles. 

Polar Distance. — The angular distance of 
a celestial body from the pole. Polar 
distances are always reckoned from the 
North pole from 0° up to 180°, by which 
means all ambiguity of expression with 
respect to the sign is avoided. 

Polaris. — Another name for the pole star. 
It is known as a Ursse Minoris, Alru- 
cabba, and Cynosura. 

Pole, Altitude of. — An arc of the meri- 
dian intercepted between the pole and 
the horizon of any place, and is equal to 
the latitude of the place. 

Pole Star, or Polar Star.— The bright star 
in the end of the tail of Ursa Minor, 
called by the names of Cynosura, Alru- 
cabba, &c. It is called the pole star be- 
cause it is within a very small angular 
distance from the pole of the sphere. It 
is also called Polaris. 

Poles. — The extremities of the axis on 
which the Earth revolves ; the poles of 
the heavens are those extremities ex- 
tended to the region of the stars. 



J 



ASTRONOMICAL DICTIONARY. 



455 



Pollux. — A name given to /? Geminorum, 
the second star in the constellation Ge- 
mini. 

Folymnia. — The thirty-third asteroid. It 
was discovered by Chacornac, October 
28, 1854. 

Pomona. — The thirty-second asteroid. It 
was discovered by Goldschmidt the 20th 
of October, 1854. 

Porrima. — A binary star in the constella- 
tion Virgo, marked y Virginis in the 
catalogues. This star is also called Post- 
varta. 

Position. — Position in the meridian is that 
adjustment of the transit-instrument 
whereby the telescope is brought into 
the plane of the meridian. 

Position, Apparent Instrumental, of an 
Object. — That position of an object in- 
dicated by an instrument, whether pro- 
perly adjusted or not. 

Position Micrometer. — See Microme- 
ter. 

Positions of the Sphere. — There are three 
positions of the sphere: Right, Parallel, 
and Oblique, which see under those 
heads. 

Position, True Instrumental, of an Ob- 
ject. — That position indicated by an 
instrument in perfect adjustment within 
itself. 

Postvarta. — See Porrima. 

Prsesepe. — A group of stars in the constel- 
lation Cancer, familiarly called the Bee- 
hive. 

Practical Astronomy. — That branch of 
astronomy which treats of astronomical 
instruments and their application. 

Praxiteles. — See Cela Sculptoria. 

Precession of the Equinoxes. — A retro- 
grade motion of the equinoctial points, 
in consequence of the action of the Sun 
and Moon upon the protuberant matter 
at the Earth's equator. 

Precession, General, of the Equinoxes.— 
The combined effect arising from the 
simultaneous action of all the celestial 
bodies, estimated in the same direc- 
tion. 

Precession, Luni-solar. — The precession 
of the equinoxes caused by the united 
attraction of the Sun and Moon only. 



Pressure. — By pressure is meant a force 
opposed by another force, so that no 
motion takes place. 

Primary Planets. — Those planets which 
revolve round the Sun as a centre, in 
contradistinction to the secondary planets, 
or satellites. 

Prime Meridian. — That meridian from 
which the longitudes of places begin to 
be reckoned. Almost every nation has 
its own prime meridian. Longitude in 
the United States is reckoned from the 
meridian of Washington, and the meri- 
dian of Greenwich is the prime meridian 
for England. 

Prime of the Moon. — The New Moon when 
she is first visible after the change. 

Prime Vertical. — That vertical circle or 
azimuth which is perpendicular to the 
meridian, and passes through the east 
and west points of the horizon. 

Priming of the Tides. — See Tides. 

Primum Mobile. — A term in the Ptolemaic 
Astronomy signifying the First mover. 
It is the highest sphere in the heavens, 
containing all the other spheres within 
it, and giving motion to them; from 
whence its name. It was supposed to 
turn round in twenty-four hours. 

Prism. — A piece of glass of any length 
having parallel sides and triangular 
ends, which separates the rays of light 
passing through it, in consequence of the 
different degrees of refrangibility that 
take place in different parts of the same 
ray. 

Prismatic Colors. — The colors manifested 
by the decomposition of a ray of light in 
passing through a prism. 

Problem. — A proposition requiring a so- 
lution. 

Procyon. — The principal star in Cania 
Minor, generally known as a Canis Mi- 
noris. 

Projectile. — Any body which, being put 
into a violent motion by an external 
force impressed upon it, is dismissed 
from the agent and left to pursue its 
course. 

Projection of the Sphere.— The repre 
sentation of the several points or places 
of the surface of the sphere, and of the 



BOUVIER S FAMILIAR ASTRONOMY. 



circles described upon it, upon a sup- 
posed transparent plane placed between 
the eye and the sphere. The projection 
of the sphere is divided into orthographic, 
stereographic, and gnomonical. 

Prolate Spheroid.— See Spheroid. 

Proper Motion. — That motion which some 
stars are found to possess, independent 
of the apparent change of place owing 
to the precession of the equinoxes, and 
which may be due either to the actual 
motion of those stars, or to the motion 
of our solar system. It is, no doubt, in 
most instances, the combined result of 
both motions. 

Proserpine. — An asteroid discovered by 
Luther, May 6, 1853. It is the twenty- 
sixth in the order of discovery. 

Prosthaphaeresis. — Another name for the 
equation of the centre or of the orbit. 
It is the difference between the true and 
mean motion or true and mean place of 
a planet. 

Psyche. — An asteroid discovered March 
16, 1852, by Gasparis. It is the six- 
teenth in the order of discovery. 

Ptolemaic System. — The arrangement of 
the heavenly bodies according to the 
theory of its founder Ptolemy. 

Puppi. — A name given to the stern of the 
ship in the constellation Argo. 

Pythagorean System. — The theory of the 
solar system, which has since been called 
the Copernican; but which differs but 
little from that of Pythagoras. 

Pyxis Nautica. — A southern constellation, 
the middle of which is R. A. 8A. 50m., 
Dec. 30° S. 

Quadrangle. — A figure having four angles. 

Quadrant. — In geometry, a quadrant is 
either the fourth part of a circle or the 
fourth part of its circumference, the arc 
of which contains 90°. It also denotes 
a mathematical instrument for taking 
the altitudes of celestial objects, and also 
for taking angles in surveying heights, 
distances, &c. The quadrant is of great 
use in astronomy and navigation. It 
was invented by Thomas Godfrey, an 
American mathematician, was taken to 
England, and has since been claimed as 



the invention of Hadley, an English 
contemporary. 

Quadrant of Altitude.— An appendix to 
the artificial globe, consisting of a thin 
slip of brass, the length of a quarter of 
one of the great circles of the globe, and 
graduated. Its use is to serve as a scale 
in measuring altitudes, amplitudes, azi- 
muths, <fec. 

Quadrature. — That aspect of the Moon 
when she is 90° distant from the Sun. 
The quadratures or quarters of the Moon 
are those two points of the Moon's orbit 
midway between the points of conjunc- 
tion and opposition. 

Quadrilateral. — A figure comprehended 
by four right lines. 

Quarters of the Heavens. — The four car- 
dinal points. 

Quarters of the Moon. — Certain periods 
in the Moon's age, known as the first 
quarter, and third or last quarter, which 
occur when she is 90° from the Sun east, 
and the same distance west of that lumi- 
nary. See Quadrature. 

Quartile. An aspect of the planets when 
they are three signs or 90° from each 
other, that distance being one-fourth 
of the whole circumference. It is known 
by the character n in astronomical 
works. 

Quintile. — An aspect of the planets when 
they are 72°, or the fifth part of the zo- 
diac, distant from each other. This term 
is now not much used. 

Radiant. — That self-luminous body which 
emits rays of light ; or an opaque body 
which reflects them. 

Radius, {radius, a ray or spoke.) — Half 
the diameter of a circle, or the distance 
from the centre to the circumference. 

Radius Vector. — An imaginary line join- 
ing the centre of a planet or comet and 
that of the Sun, or uniting the centre of 
a planet and that of its satellite. 

Rare. — Thin, not dense. 

Rasaben. — A name given to the star y 
Draconis in the Alphonsine Tables, but 
generally known by the name of Etamin. 
It passes very near the zenith of Green- 
wich, and is the star by which Bradley 



ASTRONOMICAL DICTIONARY. 



457 



made the important discovery of the 
aberration of light. 

Ras-al-asad, Australis.— A small star in 
the constellation Leo; also known as £ 
Leonis. 

Ras-al-asad, Borealis. — A star in the con- 
stellation Leo, marked in the catalogues 
// Leonis. It is also called Rasalas. 

Rasalgeti. — A bright star in the constella- 
tion Hercules ; also called a Herculis. 

Bas Alhague. — A star in the constellation 
Ophiuchus ; also called a Ophiuchi. 

Rate of the Clock. — The change of its 
error in the space of twenty-four 
hours. 

Ratio. — The relation of two magnitudes 
of the same kind in respect of quantity. 

Ray. — A beam or line of light propagated 
from a radiant point. It is also used for 
radius. 

Rays, Converging.— Those rays which 
constantly approach nearer to each other 
until they unite in one point, called the 
focus. 

Rays, Direct. — Rays of light which are in 
a straight line between the radiant point 
and the eye. 

Rays, Diverging. — Those rays which pro- 
ceed from a point and continually recede 
from each other. 

Rays, Parallel. — Those rays which con- 
tinue equally distant from each other 
throughout their whole course. Rays 
proceeding from the heavenly bodies 
are considered parallel. 

Rays, Reflected. — Rays of light which 
strike the surface of a body and are 
thrown off again. 

Rays, Refracted. — Rays of light which are 
bent out of their straight course. 

Reading Microscope. A species of com- 
pound microscope, consisting of three 
lenses, one of which is the object lens, 
and the other two constitute the eye- 
piece. This instrument is used for read- 
ing the graduated arc of a mural cir- 
cle, <fcc. 

Real Motion. — The motion of the planets 
as seen from the Sun, 

Recession of the Equinoxes.— See Pre- 
cession. 

Rectangle. — A right-angled parallelogram, 



or a right-angled quadrilateral figure; 
any solid whose angles are all right 
angles. 

Reduction. — The correction of an observa- 
tion, either for the errors of the clock, 
or for those caused by the adjustments 
of the instruments. 

Reflection. — That motion in the rays of 
light, whereby, after a near approach to 
the surfaces of solid bodies, they recede, 
or are thrown back. 

Reflecting Telescope. — Those telescopes 
which represent the images of distant 
objects by reflection. This kind of tele- 
scope is said to have been invented by 
James Gregory in 1663, the idea of 
which he received from Mersenne, who 
suggested it to Descartes as early as the 
year 16^. 

Refraction. — The bending or breaking of 
a ray of light in passing out of one me- 
dium into another of a different density. 
It is a bending of the rays of light in 
passing through our atmosphere, by 
which the apparent altitudes of the hea- 
venly bodies are increased. 

Refracting Telescope. — A telescope con- 
sisting of an object-glass inserted in one 
end of a tube, and an eye -lens or mag- 
nifying glass at the other end. 

Refrangibility. The disposition in rays 
of light to be refracted, or turned out of 
a direct line in passing out of one trans- 
parent body or medium into another. 

Regulus. — A brilliant star of the first 
magnitude in the constellation Leo ; also 
known as a Leonis or Cor Leonis. The 
name of Baailiscus was given to this star 
by Ptolemy. 

Resisting Medium. — See Medium. 

Resolvable Nebulae. — Such Nebulje as 
excite suspicion that they consist of 
stars, and which an increase of optical 
power in the telescope may bo expected 
to resolve. 

Resolution of Forces.~The resolving or 
dividing any force or motion into several 
others, in other directions, but which, 
taken together, shall have the same ef- 
fect as the single one. 

Rest. — The continuance of a body in the 
same place. It is either absolute or 



458 



BOUVIER S FAMILIAR ASTRONOMY. 



relative. Absolute rest is the continuance 
of a body in the same part of absolute 
and immovable space. Relative rest is 
its continuance in the same part of rela- 
tive space ; as, in a ship under sail, the 
continuance of a body in the same part 
of the ship is only relative rest. In re- 
ality there is no such thing as absolute 
rest, for all the heavenly bodies are sus- 
pected of motion. 

Resultant Wave.— See Wave. 

Resulting Force. — The joint eflFects of a 
number of forces united into one. 

Reticula or Reticle. — An instrument for 
measuring the quantity of eclipses with 
great nicety by means of silken threads 
attached to it. The most simple kind 
of micrometer. La Lande. 

Reticulus Rhomboidalus. — A southern 
constellation, the middle of which is R. 
A. 2,h. 44»i., Dec. 62° S. 

Retrocession of tlie Equinoxes. — See 
Precession. 

Retrogradation. — An apparent motion of 
the planets, by which they seem to go 
backwards in the ecliptic, and to move 
contrary to the order of the signs. 

Retrogradation of the Moon's Nodes. — A 
motion of the line of the nodes of the 
Moon's orbit, by which it continually 
shifts its situation from east to west, 
contrary to the order of the signs, com- 
pleting its retrograde revolution in the 
period of nineteen years. 

Retrograde Motion. — See Motiox. 

Revolution. — The motion of any body in 
a curved line until it returns to the same 
point again. The motion of a planet 
round the Sun, or of a satellite around 
its primary. 

Revolution, Annual.— The yearly motion 
of the Earth round the Sun.. 

Revolution, Anomalistic. — The same as 
Anomalistic Year, which see. 

Revolution, Diurnal. — The motion of a 
body round its own axis. 

Revolution, Nodical. — The time required 
for the node of a planet or satellite to 
occupy successively every point in its 
orbit and return again to the same place. 

Revolution, Time of. — The time a heavenly 
body requires to perform its journey 



round its centre of motion. It is syno- 
nymous with periodic time. 

Revolution, Sidereal. — The consecutive 
returns of a planet to the same star. 

Revolution, Tropical. — The consecutive 
returns of a planet to the same tropic or 
equinox. 

Rhea.— The fifth satellite of Saturn,- it 
was discovered by Dominic Cassini, De- 
cember 23, 1672. 

Rigel. — A star of the first magnitude in 
the constellation Orion; also known as 
(3 Orionis. 

Right Angle. — A right angle is formed by 
a perpendicular line falling on a hori- 
zontal one. A right angle contains 90°. 

Right- Angled Triangle.— A triangle hav- 
ing one right angle. 

Right Ascension.— The distance of any 
heavenly body measured on the equinoc- 
tial from the first point of Aries. 

Right Sphere. — That position of the 
sphere by which its poles are in the 
horizon. 

Ring. — A thin, broad circle encompassing 
the planet Saturn without touching it. 

Ring Micrometer. — A small circle of brass 
so fixed in the eye-piece of a telescope 
as to appear to the observer suspended 
in the centre of the field of view. The 
time of the appearance and disappear- 
ance of an object on the outer and inner 
edges of the ring are accurately noted, 
by which means the difference of right 
ascension and declination of two or more 
stars is ascertained. 

Rising. — The appearance of the Sun, star, 
or planet, above the horizon, which before 
was concealed beneath it. There are 
three kinds of rising, termed poetical 
rising: viz., the achronical, cosmical, 
and heliacal. 

Robur Caroli. — A southern constellation, 
the middle of which is R. A. lOh. 8m., 
Dec. 60° S. 

Rotanev. — Another name for /? Delphini, a 
star in the constellation Delphinus. 

Rotation. — The motion of a body, or sys- 
tem of bodies, about a line or point. 

Rotary. — Revolving on an axis. 

Ruchbah. — A star in the constellation Cas- 
siopeia, known also as 6 Cassiopeise. 



ASTRONOMICAL DICTIONARY. 



459 



Baclibali-ur-Raiuih. — Another name for a 
Sagittarii, a star in the constellation 
Sagittarius. 

Rudolphine Tables. — A set of mathemati- 
cal tables, published by Kepler in 1628, 
and called after the emperor Rudolph. 

Rukbat. — A double star in the constella- 
tion Cygnus ; also known as oP Cygni. 

Sabik. — Another name for a star in the 
constellation Ophiuchus ; known also as 
r) Ophiuchi. 

Sadachbia. — Another name for y Aquarii, 
a star in the constellation Aquarius. 

Sa'd-al-Melik. — A star of the third mag- 
nitude in the constellation Aquarius; 
also called a Aquarii. 

Sa'd-as-Suud. — See Kalpeny. 

Sadr. — Another name for y Cygni, a star in 
the constellation Cygnus. 

Sagitta. — One of the northern constella- 
tions, the middle of which is R. A. 19A. 
48m., Dec. 18° N. 

Sagittarius. — One of the zodiacal constel- 
lations, the middle of which is R. A. 19A. 
0»j., Dec. 30° 0' S. 

Saiph. — A star in the constellation Orion ; 
also called x Orionis. 

Saros. — The period of 6585-78 days, in 
which eclipses occur again very nearly 
in the same order within the same 
period. It was called Saros by the 
Chaldean and Egyptian astronomers. It 
is also known as the Period of Eclipses. 

Satellite. — Secondary planet or Moon re- 
volving around another planet as its 
centre, as the Moon revolves round the 
Earth. 

Saturn. — The next planet beyond Jupiter. 
By the Hebrews he was called Shebtai, 
rest, and by the Greeks xpoi^og, time ; be- 
cause he is so long a time in performing 
his journey round the Sun. 

Saturnicentric. — As seen from, or having 
relation to, the centre of the planet Sa- 
turn. 

Scale. — The degrees and minutes of an arc 
of a circle, or of any right line drawn 
or engraved upon a ruler. 

Scbeat. — One of the stars in the constella- 
tion Pegasus ; also called /? Pegasi, 
though the Arabs called it Monkib. It 



is the north-western star in the Square 
of Pegasus. The name of Scheat is also 
given to 6 Aquarii, a star in the constel- 
lation Aquarius. 

Schedir. — A star of the third magnitude ; 
also called a Cassiopeise. 

Schemali. — A star in the extremity of the 
tail of Cetus ; also termed t Ceti. 

Scintillation. — The twinkling or tremu- 
lous motion of the light of the fixed 
stars, supposed to be due to the inter- 
ference of light, 

Scorpio. — One of the zodiacal constella- 
tions, the middle of which is R. A. 16h. 
19m., Dec. 26° N. 

Scutum Sobieski. — One of the northern 
constellations, the middle of which is R. 
A. ISh. Z2m., Dec. 10° S. 

Sea Astrolabe. — An instrument used for 
taking altitudes at sea; as the altitude 
of the pole, the Sun, or the stars. 

Seasons. — The four portions or quarters of 
the year, distinguished by the names of 
Spring, Summer, Autumn, and Winter. 

Secant. — A line cutting a circle lying 
partly within and partly without it. 

Second. — In time, it is the 60th part of a 
minute, the 3600th part of an hour, and 
the 86,400th part of a day. In angular 
measurement, it is equal to the 60th part 
of a minute, the 3600th part of a degree, 
and the 1,296,000th part of the whole 
circumference. 

Secondary Planet. — See Satellite. 

Secular Acceleration. — A slow change in 
the eccentricity of the Earth's orbit, 
which has sensibly diminished the 
length of the Moon's revolution since the 
time of the earliest observations. The 
eccentricity of the Earth's orbit is de- 
creasing at the rate of about forty miles 
annually; and, were it to decrease at 
that rate steadily, it would require 
39,861 years to bring it to a circle. 

Sector. — That portion of a circle included 
between two radii and their included arc. 

Sector, Astronomical. — An instrument for 
finding the difference in right ascension 
and declination bet^^'cen two objects 
whose distance is too great to be mea- 
sured by moans of a micrometer in a 
fixed telescope. 



X 



460 



BOUVIER S FAMILIAR ASTRONOMY. 



Sector of a Sphere. — The conic solid whose 
vertex ends in the centre and its base 
in the segment of the same sphere. 

Secular Equations, or Century Equations. 
— Corrections required to compensate 
such inequalities in the celestial motions 
as occur in the course of a century, or 
one hundred years. 

Secular Inequalities.— Variations in the 
motions of the heavenly bodies, requiring 
an indefinite period for their accomplish- 
ment, sometimes amounting to many 
centuries. 

Segment. — A part cut oflf a figure by a 
line or plane. The part remaining after 
the segment is cut off is called a frus- 
tum. 

Selenocentric. — As seen from, or having 
relation to, the centre of the Moon. 

Selenography, {ccXrivri, the Moon, and ypa<pco, 
to describe.) — The description and re- 
presentation of the Moon, with all the 
parts and appearances of her disc or 
face j as geography is of the surface of 
the Earth. 

Semicircle. — Half a circle, or a segment 
cut off by a diameter. 

Semidiameter. — Half the diameter of a 
sphere ; the radius. 

Semi-Diurnal Arc. — One half the arc de- 
scribed by a heavenly body between its 
rising and setting. 

Semi-Sextile. — An aspect of two planets 
when they are distant from each other 
30° or one sign. 

Sensible Horizon. — See Horizon. 

September, {septem, seven.) — In the Ro- 
man year, this was the seventh month; 
but is the ninth according to our reck- 
oning. 

Sequences. — A term employed by Sir J. 
Herschel in some of his tables, to signify 
the gradual diminution of the apparent 
magnitude of the individuals of certain 
series of stars when compared with each 
other. 

Serpens. — One of the northern constella- 
tions, the middle of which is in R. A. 
llh. Om., Dec. 3° 0' N. 

Serpha. — See Denebola. 

Serpentarius. — Another name for Ojahiu- 
chus, which see. 



Sesquiplicate. — Having the ratio of two 
and a half to one. 

Setting. — The descent of the Sun, star, or 
planet, below the horizon. 

Seven Stars. — See Pleiades. 

Sexagesimal. — The division of the circle 
by sixties. — Thus, the circumference is 
divided into 360 equal parts called de- 
grees, and each degree into 60 equal 
parts called minutes, and each minute 
into 60 equal parts called seconds. 

Sextans. — A southern constellation, the 
middle of which is R. A. 10^. Om., Dec. 0°. 

Sextant. — The sixth part of a circle ; an 
astronomical instrument. 

Sextile. — The aspect or position of two 
planets when they are distant the sixth 
part of the circle or 60° ; it is marked 
thus *. 

Sheliak. — A star in the constellation Lyra; 
also known as jS Lyrae. 

Sheratan. — A star in the constellation 
Aries, called Arietis. 

Shooting Stars. — See Meteor. 

Sidereal, [sidus, a star.) — Relating to the 
stars. 

Sidereal Astronomy. — That branch of 
astronomy which treats of the stars. 

Sidereal Clock. — A clock which marks 
sidereal time ; that is, which goes at 
such a rate as to show Oh, Om. Os, when 
the equinox comes to the meridian. 
This is an indispensable piece of furni- 
ture in every observatory. 

Sidereal Day. — The interval of time be- 
tween two consecutive passages of the 
meridian over the mean equinox. 

Sidereal Day, Apparent. The interval of 
time between two consecutive passages 
of the meridian over the apparent equi- 
nox. 

Sidereal Time.— See Time. 

Sidereal Year. — See Year. 

Sidus, or Georgium Sidus. — The name 
given to Uranus by its discoverer. Dr. 
Wm. Herschel. 

Sign. — A twelfth part of the ecliptic or 
zodiac, containing 30°. 

Signs. — See Characters. 

Sine. — The perpendicular drawn from the 
extremity of an arc to the diameter ot 
a circle. 



ASTRONOMICAL DICTIONARY. 



461 



Sirius. — The brightest star in the firma- 
ment ; also called a Canis Majoris. The 
Egyptians are supposed to have given 
the name of Sirius to this star from Siris, 
which was one of the names of the river 
Nile ; for that stream began to swell at 
that time of year when Sirius rose with 
the Sun, Sirius is also known by the 
name of the Bog Star; and it is also 
called Canicula. 

Si tula. — A name given to a star in the 
constellation Aquarius, called also k 
Aquarii. This star was called by the 
Arabs Al-Delio. 

Solar, {Sol, the Sun.) — Relating to the Sun. 

Solar Apex. — The point in space toward 
which the Sun is moving. 

Solar Day. — See Day. 

Solar Spectrum. — The oblong image form- 
ed by the prism, and divided into seven 
colored bands or spaces. 

Solar System. — The Sun and the bodies 
which revolve around him. 

Solar Time, Apparent.— Time measured 
by the hour angle of the Sun. 

Solar Year. — See Year, Tropical. 

Solstices, {Sol, the Sun, and siato, to stand.) 
— The points in which the Sun is farthest 
from the equator; at which time he ap- 
pears to stand still for a few days. 

Solstitial Colure. — The great circle which 
passes through the solstitial points. 

Solstitial Points. — The points in the eclip- 
tic which designate the Sun's place at 
the time of the solstices ; they are the 
first points of Cancer and Capricorn, 
and are 90° from the equinoctial points. 

South. — One of the four cardinal points, 
being that directly opposite to the north. 
A star, planet, or the Moon, is said to be 
south when it crosses the meridian south 
of the observer. 

Southing. — The passing of any celestial 
body over the meridian. It is particu- 
larly used in regard to the Moon. 

Southern Constellations. — See Constel- 
lations. 

Southern Signs. — Those six signs of the 
zodiac situated south of the equator : 
viz., Libra, Scorpio, Sagittarius, Capri- 
cornus, Aquarius, and Pisces. 

Space. — Infinite extension in all directions. 



Spectrum, {specto, to behold.) — The image 
formed on any white surface by a ray of 
solar light passing through a small hole 
into a dark chamber, when refracted by 
a triangular glass prism. The ray is 
divided into seven bands exhibiting the 
seven colors observable in the rainbow ; 
the image is called the spectrum, and, be- 
cause it is produced by means of a prism, 
it is also termed the prismatic spectrum. 
The colors forming the spectrum are 
the prismatic colors. 

Speculum, {specio, to view.) — A mirror; a 
metallic reflector used in reflecting tele- 
scopes, instead of the object-glass used 
in refracting telscopes; any polished 
body impervious to the rays of light. 

Sphere. — A solid body contained under 
one single uniform surface, every point 
of which is equally distant from a point 
in the middle called its centre. 

Sphere, Armillary, {armilla, a bracelet or 
ring for the arm.) — An astronomical in- 
strument representing the several circles 
of the sphere. They are made of rings 
of brass, serving to illustrate the posi- 
tion of the difi"erent circles, and to solve 
the various problems relating to them. 

Sphere, Ohlique. — That position of the 
Earth when the rational horizon cuts the 
equator obliquely. All the inhabitants 
of the Earth, except those who live at 
the poles or at the equator, have this 
position of the sphere. 

Sphere, Parallel. — That position in which 
the rational horizon coincides with the 
equator, the zenith and nadir being the 
poles. The inhabitants, if any, who 
have this position of the sphere are 
those who live at the poles. 

Sphere, Right. — That which cuts the 
equator at right angles, or that which 
has the poles in the horizon and the 
equinoctial in the zenith. The inhabit- 
ants at the equator have this position 
of the sphere. 

Spheroid. — A solid body approaching to 
the figure of a sphere, but having one 
of its diameters longer than the other. 

Spheroid. Oblate. — A figure formed by the 
rotation of an ellipse about the shorter 
axis. 



462 



BOUVIER S FAMILIAR ASTRONOMY. 



Spheroid, Prolate. — A figure produced by 
the revolution of a semi-ellipse about its 
longer diameter. 

Spica. — Another name for the star a Yir- 
ginis, belonging to the constellation 
Virgo. It was called As-SimaJc by the 
Arabs, and its Nubian name was Ele- 
azelet. 

Spiral Nebulae.— See Xebul^. 

Sporades, (from c-tifM, to scatter.) — A name 
given by astronomers formerly to such 
stars as were not included in any con- 
stellation; they are now caMQd, unformed 
stars. 

Spots. — Certain portions of the Sun's or 
Moon's disc, observed to be either 
brighter or darker than the rest. See 
Faculx and Macul.e. 

Spring Tides. — The high tides about the 
times of Xew and Full Moon. 

Star. — A general name for most of the 
heavenly bodies. 

Star-Dust. — A name applied to patches of 
extremely minute stars, as seen through 
the telescope. 

Stars, Fixed. — Those celestial bodies 
which retain the same, or nearly the 
same, positions with regard to each 
other. 

Stars, Temporary.— Those stars which 
appear suddenly, and vanish after being 
visible a short time. 

Stars, Variable. — Those stars which ex- 
hibit periodical fluctuations of bril- 
liancy. 

Stationary. — The state of a planet when, 
to an observer on the Earth, it appears 
for some time to stand still, because it is 
moving in a line with the eye. 

Stellar. — Relating to the stars. 

Style. — A particular manner of computing 
time, as Old Style and Xeic Style. 

Style, Old. — The Julian manner of reck- 
oning, as instituted by Julius Caesar. 

Style, New. — the Gregorian manner of 
computing, as instituted by Pope Gre- 
gory XIII. in the year 1582. 

Sub- Polar. — Below the pole ,; on the lower 
meridian. 

Succession of the Signs. — That order in 
which they are commonly reckoned, as 
Aries, Taurus, Gemini, Cancer, &c. 



Sulaphat. — A bright star in the constella- 
tion Lyra ; also known as y Lyrae. 

Sun. — The great centre of the solar sys- 
tem. 

Sun's Horizontal Parallax. — The greatest 
angle under which the equatorial semi- 
diameter of the Earth would appear at 
the Sun's centre. 

Sun's Mean Longitude. — The arc of the 
ecliptic from the vernal equinox to the 
place the Sun would appear to occupy, 
had his motion been uniform and equal. 

Sunday Letter. — See Domixical Letter. 

Superior Conjunction. — See Coxjuxctiox. 

Superior Planets. — The same as the exte- 
rior planets. 

Superior Meridian. — That half of the 
meridian above the horizon. 

Svalocin. — A star in the constellation Del- 
phinus, called also a Delphini. 

Sweep. — A word applied to an examina- 
tion of the sky by " sweeping" the tele- 
scope over a long extent of the heavens, 
and thus observing a band whose breadth 
is the width of the field of view em- 
braced in the instrument, and whose 
length is the length of the arc of the 
heavens surveyed. This method of ob- 
serving is accomplished with great fa- 
cility by clamping the telescope, and 
allowing the stars to move through the 
field. In this case the length of the 
sweep is measured by the length of time 
the eye is at the instrument ; and, by 
changing the altitude of tbe telescope 
after each sweep, so that the lower edge 
of the field of one observation shall be 
the upper edge of the other, the whole 
sky may be examined. Gould. 

Symbol, Astronomical. — See Characters. 

Sjrnodic Revolution. — The time between 
two conjunctions or two oppositions of 
the Moon or a planet. 

Synodical Month. — See LrxATiox. 

System. — An hypothesis or a supposition 
of a certain order and arrangement of 
the several parts of the universe, by 
which astronomers explain all the phe- 
nomena or appearances of the heavenly 
bodies ; a number of bodies revolving 
round a common centre. The principal 
systems which have been embraced are 



ASTRONOMICAL DICTIONARY. 



463 



four; namely, the Ptolemaic, the Egyp- 
tian, the Tychonic, and the Copernican. 

System, Copernican. — That system of 
the universe as taught by the astrono- 
mers of the present day. It was first 
promulgated by Pythagoras 2300 years 
ago, and was again revived in the fifteenth 
century by Nicholaus Copernicus. 

System, Egyptian. — That system of the 
universe taught by the Egyptian astro- 
nomers, in which the Earth is the cen- 
tre, and the Sun and Moon revolve 
around it. The planets and Mercury 
and Venus revolve round the Sun, and 
the exterior planets in orbits far distant. 

System, Ptolemaic. — That system of the 
universe taught by Ptolemy. It con- 
sisted of the Earth in the centre, and 
the Moon, Mercury, Venus, the Sun, 
Mars, Jupiter, and Saturn, revolving 
round it in the above-named order. 

System, Tychonic. — That system of astro- 
nomy taught by Tycho Brahe. It 
consisted of the Earth in the centre, 
with the Moon revolving round it, and 
the Sun as the centre of the orbits of 
the other planets. The Sun, according 
to this system, was supposed to revolve 
round the Earth. 

Syzigies, (from o-v^eu^ij, conjunction.) — 
Those points in the Moon's orbit where 
she is new and full. 

Tables. — Astronomical tables are the com- 
putations of the motions, places, and 
other phenomena of the heavenly bodies. 

Tables, Alphonsine. — A set of astrono- 
mical tables published about the year 
1250, under the patronage of Alphonso 
X., King of Castile and Leon. 

Tables, Danish.— Tables of the motions 
of the Sun, Moon, and planets, by Lon- 
gomontanus, from the observations of 
Tycho Brahe. 

Tables, Ludovician. — Tables composed by 
De la Hire, in 1702. The observations 
were entirely his own, without the 
assistance of any hypothesis ; which, 
before the invention of the micrometer 
and pendulum clock, was deemed impos- 
sible. 

Tables, Prutenic. — Tables compiled by 



Erasmus Reinhold from the observations 
and calculations of Copernicus. 

Tables, KudolpMne. — Astronomical tables 
made by Kepler from the observations 
of Tycho and his own. 

Tabulae Toledanse. — A set of astronomical 
tables made by Arzachel, an Arabian 
astronomer, in 1020. 

Tail of a Comet. — The train of luminous 
light which shoots out from a comet. 

Talita. — A star in Ursa Major, called t Ursse 
Majoris. 

Tangent, {tango, to touch.) — A line that 
touches a curve; that is, which meets it 
in a point without cutting it, though it 
be produced both ways. 

Tangent Screw. — A screw which works 
tangentially, imparting a slight motion 
to an astronomical instrument. 

Tangential Force. — A force exerted in the 
direction of a tangent. 

Tarandus. — A northern constellation, the 
middle of which is situated R. A. 2Ji. 
32m., Dec. 80° N. 

Tarazed. — A star in the constellation 
Aquila; also known as y Aquilse. 

Taurus. — One of the twelve signs of the 
zodiac, and the second in order, the mid- 
dle of which is R. A. 4h. 20m., Dec. 
16° N. 

Taurus Poniatowski. — A northern con- 
stellation, situated in R. A. ISh. 20m., 
Dec. 7° N. 

Taygeta. — A name given to one of the 
Pleiades. 

Tegmine. — A triple star in the constella- 
tion Cancer ; called also ^ Cancri. 

Telescope, {Tv\£,far; in the distance; and 
cKOTreo), to observe.) — An optical instru- 
ment which serves for discovering and 
viewing distant objects, either directly 
by glasses, or by reflection by means of 
speculas or mirrors. 

Telescope, Achromatic, (a, deprived of, and 
Kpo^a, color.) — A telescope invented by 
Mr. Dollond, and so contrived as to re- 
medy the aberration arising from colors. 

Telescope, Meridian, or Transit Instru- 
ment. — See Transit Instrument. 

Telescopic Comets. — Those comets which 
are invisible without the aid of a tele- 
scope. 



464 



BOUVIER S FAMILIAR ASTRONOMY. 



Telescopic Planets. — The asteroids, or, as 
they are sometimes called, the Planet- 
oids, are also known as the telescopic 
planets. The planet Neptune cannot be 
seen without the aid of a telescope. 

Telescopic Stars. — Those stars which are 
invisible without the aid of a tele- 
scope. 

Telescopium. — A southern constellation, 
the middle of which is R. A. ISA. 20»i., 
Dec. 52° S. 

Temperate Zone. — That zone or space 
situated between the tropics and polar 
circles. 

Temporary Stars. — Stars which have sud- 
denly appeared, shining with great bril- 
liancy, and after. a time disappear en- 
tirely. 

Terminator. — That line which separates 
the illuminated from the unilluminated 
portion of the Moon's disc. 

Terrestrial, {terra, the Earth.) — Apper- 
taining to the Earth. 

Tethys. — The third satellite of Saturn. It 
was discovered by Dominic Cassini in 
March, 1684. 

Thalia. — An asteroid discovered by Hind, 
December 15, 1852. It is the twenty- 
third in the order of discovery. 

Thebit. — An inequality on the lunar sur- 
face is known by this name. 

Theemim. — Another name for v Eridani, 
a star in the constellation Eridanus. 

Themis. — An asteroid discovered by Gas- 
paris, August 5, 1853. It is the twenty- 
fourth in the order of discovery. 

Thermometer, (^ws, T^eat, and nerpov, a 
measure.) — An instrument for measuring 
the temperature of the air, water, <fec. 

Thermoscope. — A species of thermometer. 

Thetis. — An asteroid discovered by Luther, 
April 17, 1852. It is the seventeenth in 
the order of discovery. 

Thuban. — A star in the constellation Draco, 
known also as a Draconis. About 4600 
years ago it was the north pole star. 

Tide, (from the Saxon tid, which signifies 
time or season.) — Two periodical motions 
of the waters of the sea, produced by the 
action of the Sun and Moon. They are 
called the flux and reflux, or ehh and 
flA>od, 



Tide-Day. — The interval between the oc- 
currences of two consecutive maxima of 
the resultant \oave at the same place. 
The tide-day varies as the component 
waves approach to, or recede from, one 
another. 

Tide-Gauge. — An instrument employed for 
discovering the height of the tides. 

Tides, Apogean. — See Apogean. 

Tides, Atmospheric. — Atmospheric tides 
are certain periodical changes in the 
atmosphere, similar, in some respects, to 
those which take place in the ocean, and 
produced, in a measure, by the same 
causes. 

Tides, Derivative Ebb. — Low water suc- 
ceeding the apex of the tide-wave after 
it has passed onward and the water flows 
out into the sea. 

Tides, Derivative Flood. — The approach 
of the tide-wave along shallow shores 
and beds of estuaries, intercepting the 
free transmission of the tide and causing 
it to be heaped up in the ocean, pro- 
duces derivative flood-tides. 

Tides, Lagging of. — The variation in the 
length of the tide-day, indicating a 
retardation of the recurrence of high 
water at any given place. 

Tides, Perigean.— See Perigean. 

Tides, Priming of. — The variation in the 
length of the tide-day, owing to the 
acceleration of the recurrence of high 
water at any given place. 

Tide-Tables. — Tables exhibiting the time 
of high water at sundry places. 

Tide-Wave, Derivative.— The swell in 
rivers and channels produced by the 
primitive tide-wave, or general swell of 
the ocean. 

Tide-Wave, Primitive. — The general swell 
of the ocean. 

Time. — Duration marked by the motion 
of the heavenly bodies, particularly that 
of the Earth on its axis, being the most 
regular occurrence which comes under 
the notice of man. See Mean Time, and 
Day, Mean Solar. 

Time, Apparent. — Time denoted by an 
observation of the Sun ; that is, by the 
arrival of the real Sun at the meridiar, 
and is the time indicated by dials. 



ASTRONOMICAL DICTIONARY. 



465 



Time, Equinoctial. — The mean longitude 
of the Sun converted into time at the 
rate of 360° to the tropical year. A 
method of reckoning time for astrono- 
mical purposes, called equinoctial time, 
was suggested by Sir John Herschel. 
Its object is to avoid the necessity of men- 
tioning the place to which the time of an 
observation refers, for any particular 
hour mean time at Greenwich is not the 
same hour at that moment on any other 
meridian, but differs therefrom by the lon- 
gitude of the place. Sir John Herschel 
proposed the moment of the vernal equi- 
nox as a starting-point for the reckoning 
of time which would be common to all 
nations. Hind. 

Time, Mean. — Time denoted by the mean 
Sun ; that is, time reckoned by the ar- 
rival of an imaginary sun at the meri- 
dian, or of one which is supposed to 
move uniformly in the equinoctial at the 
rate of 59' 8""33 every day, which is the 
time given by clocks and watches in 
common life. 

Time, Sidereal. — The time shown by a 
clock regulated by the fixed stars. 

Titan. — The sixth satellite of Saturn. It 
was discovered by Huyghens, March 25, 
1655. 

Titania. — A name given by Sir John 
Herschel to the fourth satellite of Ura- 
nus. 

Torrid Zone, (torreo, to roast.) — That zone 
or space situated between the tropics, 
over some part of which the sun is al- 
ways vertical. 

Total Eclipse.— The entire obscuration of 
a celestial object by a body or shadow 
of another body. 

Toucan. — A southern constellation, the 
middle of which is in R. A. 23h. 55m., 
Dec. 66° S. 

Trajectory. — A curvilinear path described 
by a body, as the orbit of a comet. 

Transit, {tram, beyond, and eo, to go.) — 
The passage of any planet just before 
or over the Sun, another planet, or a star, 
or the passing of a heavenly body over 
the meridian or across the field of a 
telescope. 

Transit Circle. — An instrument so con- 



structed that the right ascension and 
declination of a heavenly body may be 
determined at its transit over the meri- 
dian. 

Transit Instrument. — A telescope fixed at 
right angles to a horizontal axis; this 
axis being so supported that the line of 
collimation may move in the plane of 
the meridian; a telescope so mounted 
as to observe the passage of a heavenly 
body across the meridian. 

Transits of Satellites. — The passage of 
the shadows of Jupiter's satellites across 
his disc, which to the inhabitants of 
Jupiter produce eclipses of the Sun. 
Sometimes these satellites have been 
seen to transit his disc like beads of light. 

Transit, Upper. — The passage of a star 
across the upper or superior meridian of 
any place; that is, the passage across 
the great circle passing through the 
zenith of the place and the north and 
south points of the horizon. 

Transit, Lower. — The passage of a hea- 
venly body over the lower or inferior 
meridian of the place; that is, over the 
meridian differing 180° from the longi- 
tude of the place. 

Transparent, (trans, through, and pareo, 
to appear.) — That condition of a body 
which permits light to pass through it 
freely. 

Transverse Axis. — See Axis. 

Triangle, {rpsts, three.) — A figure bounded 
by three straight lines, and which conse- 
quently has three angles, from which 
the figure takes its name. 

Triangulum. — A northern constellation, 
the middle of which is in R. A. 1^. 4:8m., 
Dec. 32° N. 

Triangulum Australis. — A southern con- 
stellation, R. A. 16h. Qm., Dec. 65° S. 

Triangulum Minus. — A northern constel- 
lation, the middle of which is situated 
in R. A. Ih. 28m., Dec. 27° N. 

Trine. — The aspect or situation of one 
planet with respect to another, when 
they are distant 120°, or the third part 
of the circle. 

Triones. — The assemblage of seven stars 
in the constellation Ursa Major, popu- 
larly called Charleii'a Wain, 



466 



BOUVIER S FAMILIAR ASTRONOMY. 



Triple Stars. — Three stars forming a sys- 
tem and revolving about one common 
centre of gravity. 

Triquetrum. — An ancient astronomical in- 
strument, supposed to have been invented 
by Ptolemy for determining the altitudes 
and amplitudes of the heavenly bodies. 

Tropical Revolution of the Moon.— The 
passage of the Moon from one longitude 
to the same longitude again. 

Tychonic or Tychonean System. — That 
system of the universe as taught by 
Tycho Brahe. 

Tropical Year.— See Year. 

Tropics, (rpCTO), to turn ; because when the 
Sun arrives at these points he seems to 
turn or bend his course.) — Two circles 
situated 23° 28' on each side of the 
equator, and between which the Sun is 
always vertical. 

True Anomaly. — See Anomaly. 

True Motion. — See Motion. 

True Place of a Star or Planet.— A point 
in the heavens shown by a right line 
drawn from the centre of the Earth 
through the centre of the star or planet. 

Tureis. — A star in the constellation Argo 
Navis, situated near the tropic of Capri- 
corn, called I Argo Navis. 

Twilight. — The faint light seen before sun- 
rise and after sunset; the crepusculum. 

Tycho. — A lunar mountain. 

Ultra Zodiacal Planets.— A term fre- 
quently api>lied to the asteroids ; those 
planets whose orbits are not within the 
limits of the zodiac. 

TJmhra. — A shadow. 

Umbriel. — A name given by Sir John Her- 
schel to the second satellite of Uranus. 

Unformed Stars. — See Sporades. 

Unicorn. — See Monoceros. 

Universe, (»»«»«, one, and versum, turn- 
ing.) — A name applied by the ancients 
to the whole of the heavens and Earth. 

Unuk-al-Hay. — Another name for the prin- 
cipal star in the constellation Serpens, 
known also as a Serpentis. 

Urania. — The thirtieth asteroid. It was 
discovered by Hind, July 22, 1854. 

Uranihurg. — The name of a celebrated 
observatory, in which Tycho Brahe com- 



posed his catalogue of stars. Lat. 55° 
54' N., Ion. 12° 47' E. from Greenwich. 

Uranography, {ovpavo^, the heavens, and 
ypa<p(i}, to describe.) — A description of the 
heavens and the heavenly bodies. 

Uranometry, {ovpavo;, the heavens, and nerpov, 
a measure.) — The measurement of the 
heavens. 

Uranus. — One of the planets of the solar 
system, situated next beyond the orbit 
of Saturn. It was discovered by Sir W. 
Herschel, March 13, 1781, and by him 
called Georgium Sidus. 

Urkab-ur-Ramih. — A name given by the 
Arabians to the star /? Sagittarii, in the 
constellation Sagittarius. 

Ursa Major. — A northern constellation, the 
middle of which is R. A. lOh. 30m., Dec. 
60° 0' N. 

Ursa Minor.— It is also called Arctos Mi- 
nor; one of the northern constellations, 
the middle of which is R. A. 15h. BOm., 
Dec. 75° N. 

Variable Stars.— Stars which exhibit pe- 
riodic changes of brilliancy. 

Variation, Annual.— The diflFerence in the 
right ascension or declination of a star 
produced by the combined effect of the 
precession of the equinoxes and proper 
motion of the star. 

Variation of the Moon. — The third in- 
equality observed in the Moon's motion, 
by which, when out of her quadratures, 
her true place differs from her place 
twice equated. 

Vector. — See Radius Vector. 

Vega. — A bright star of the first magni- 
tude in the constellation Lyra; also 
called a Lyrse, or Wega. 

Venus. — One of the interior planets, and 
second in order from the Sun. 

Velis. — A name given to the sails of the 
ship in the constellation Argo. 

Vernier. — A contrivance for measuring in- 
tervals between the divisions of gradu- 
ated scales or circular instruments. It 
was invented by Peter Vernier. 

Vertex. — The zenith; the top of anything 
ending in a point. 

Vertical, {verto, to turn.) — A position at 
right angles to the plane of the horizon. 



ASTRONOMICAL DICTIONARY. 



467 



Vertical Circle. — A great circle of the 
sphere, passing through the zenith and 
nadir. 

Vertical Diameter of the Sun or Moon. — 
The diflference of the zenith distances of 
the north and south limbs, corrected for 
refraction and parallax. 

Vertical Points. — The zenith and nadir. 

Vertical, Prime. — See Prime Vertical. 

Vertical of the Sun.— The vertical circle 
which passes through the centre of the 
Sun at any moment of time. 

Vesta. — One of the asteroids discovered by 
Dr. Olbers, March 29, 1807. It is the 
fourth in the order of discovery. 

Via Lactea. — See Galaxy. 

Via Solis. — The ecliptic ; the Sun's path. 

Victoria. — The twelfth asteroid, sometimes 
called Clio. 

Vildiur. — A star in the constellation Ursa 
Minor; also known as i Ursae Minoris. 

Vindemiatrix. — A star in the constellation 
Virgo, marked e Virginis in the cata- 
logues. 

Virgo. — One of the zodiacal constellations, 
the middle of which is R. A. \Zh. 14m., 
Dec. 6° N. 

Virgula. — An instrument used by Huy- 
ghcns to measure the discs of the Sun, 
Moon, and planets. It consisted of a 
tapering piece of metal interposed in the 
common focus of the eye-lens and ob- 
ject-glass of a telescope, and which was 
made to slide across the tube. 

Vis Inertia. — Immobility without force; 
a power which, according to Newton, is 
implanted in all matter, of resisting any 
change from a state of rest. 

Vitello. — A spot on the surface of the 
Moon called by that name. 

Volume. — Dimensions; space occupied; 
bulk. 

Volume of a Body. — The number of cubic 
units which it contains; or the number 
of times a solid contains another solid, 
taken as a unit of measure. 

Vulpecula et Anser. — One of the northern 
constellations, the middle of which is R. 
A. 2(ih. Om., Dec. 23° N. 

Wasat. — Another name for the star 6 Ge- 
minorum, in the constellation Gemini. 



Wave, Resultant. — That wave whose apex 
is at some intermediate place between 
the apexes formed by the action of the 
Sun and Moon, which depends upon the 
angular distances of those two lumina- 
ries. 

Week. — A period of time, consisting of 
seven days, which dates its origin from 
the earliest antiquity, being used by the 
ancient Syrians, Egyptians, and most of 
the oriental nations. 

Wega, or Vega. — A star of the first mag- 
nitude in the constellation Lyra, and 
known as a Lyrae. 

Weight. — That property of bodies by 
which they tend to the centre of the 
Earth; gravity. The term weight is 
also used to express the relative value or 
importance of observations or determina- 
tions. Thus, an observation or mea- 
surement, whose weight on account of its 
accuracy is designated as two, is said to 
be equal to two observations or measure- 
ments the weight of each of which is 
only one. The rules of the weights of a 
series of observations are determined by 
multiplying each observation by its re- 
spective weight, adding the products, 
and dividing by the mean of the weights, 
instead of the arithmetical mean. Gould. 

West. — One of the four cardinal points of 
the horizon, towards which the heavenly 
bodies set. 

Wezen. — A star in the constellation Canis 
Major, called in the catalogues J Canis 
Majoris. 

Wire Micrometer. — A micrometer con- 
taining vertical and horizontal wires, 
much used in astronomical observations. 

Wyes. — The supports of the telescope of 
the transit instrument, theodolite, «fec. 

Xiphias. — Another name for Dorado, 
which see. 

Y's.— See Wyes. 

Year. — The time the Earth tnkes to per- 
form her journey round the Sun. 

Year, Anomalistic. — The time between 
two successive passages of the Earth 
through its aphelion or perihelion 
points. 



468 



BOUVIER S FAMILIAR ASTRONOMY. 



Year, Bissextile. — See Bissextile. 

Year, Common. — That year which consists 
of 365 days, in contradistinction to the 
bissextile year, which consists of 366. 

Year, Sidereal. — The time which the Sun 
takes in passing from any fixed star till 
he returns to it again is called a sidereal 
year. 

Year, Tropical.— The time which the Sun 
takes in moving through the ecliptic from 
one tropic or equinox till it returns to it 
again is called a tropical or solar year. 

Yed Posterior. — The name given to the 
eastern one of two stars called Yed in 
the constellation Ophiuchus, marked in 
the catalogues s Ophiuchi. 

Yed Prior. — A star in the constellation 
Ophiuchus, called also 6 Ophiuchi. 

Zaurak. — The name of a star in the con- 
stellation Eridanus ; also known as y 
Eridani. 

Zavijava. — A star in the constellation Vir- 
go, marked /? Virginis in the catalogues. 

Zenith. — The vertical point, or point in the 
heavens directly overhead. 

Zenith, Apparent. — The point in which a 
plumb-line produced intersects the celes- 
tial sphere overhead. 

Zenith, Central. — That point in which the 
radius of the Earth produced through 
the spectator's place intersects the celes- 
' tial sphere. 

Zenith Distance. — The distance of the Sun 
or a star from our zenith. The comple- 
ment of the altitude, or what it wants' 
of 90°. 

Zodiac, {^(^ov, an animal; because the zo- 
diacal constellations have the forms of 
animals given to them.) — A broad belt 
or zone in the heavens, within which all 



the orbits of the planets may be found 
except some of the asteroids. It ex- 
tends to 8° on each side of the ecliptic. 

Zodiacal Constellations. — Those twelve 
constellations through which the ecliptic 
passes,and through which the Sun appears 
to move. They are Aries, Taurus, Ge- 
mini, Cancer, Leo, Virgo, Libra, Scorpio, 
Sagittarius, Capricornus, Aquarius, and 
Pisces. 

Zodiacal Light. — A faint luminosity of a 
conical form which may sometimes be 
seen in the horizon after twilight. 

Zodiacal Planets. — Those planets whose 
orbits lie within the limits of the zodiac. 

Zodiacal Signs. — Certain astronomical 
characters which have been given to the 
zodiacal constellations. See Charac- 
ters. 

Zone. — A division of the Earth's surface 
by means of parallel circles. There are 
five zones : one ton-id, two temperate, 
and two frigid. The father of the Greek 
philosopher, Thales, was the first person 
who divided the sphere into zones. 

Zone of Declination.— A portion of the 
celestial sphere comprised between cer- 
tain parallels of declination. 

Zosma. — A star in the constellation Leo; 
also called 6 Leonis. 

Zuben-el-Akrab. — A star in the constella- 
tion Libra, called also rj Librae. 

Zuben-el-Gemabi. — A star in the constel- 
lation Libra ; also known as /? Librae. It 
is sometimes called Kiffa Borealis in 
catalogues. 

Zuben-el-Gubi. — A small star in the con- 
stellation Libra; it is also known as y 
Librae. 

Zuben-es-Chamali. — A star in Libra; also 
known as a Librae, or Kiffa Australia. 



TABLES. 



TABLE L 

ELEMENTS OF THE PLANETARY SYSTEM. 



Sun 

Mercury . 
Venus.... 

Earth 

*Mars.... 
Jupiter.. 
Saturn... 
Uranus .. 
Neptune , 



Inclination 

of Orbit 
to Ecliptic. 



7 9.1 
3 23 28.5 



1 51 06.2 

1 18 5.1.3 

2 29 35.7 

46 28.4 

1 46 59.0 



Inclination 

of Axis to 

Orbit. 



7 30 00.0 

unknown. 
73 32 00.0 
23 27 56.5 

30 18 10.8 
3 05 30.0 

31 19 00.0 
00 00.0 

unknown. 



Mass, Sun 


Density, 


as 


Earth as 


Unity. 


Unity. 


1 


0.25 


4865751 


1.12 


401839 


0.92 


389551 


1.00 


2680337 


0.95 


1047.871 


0.24 


3501.600 


0.14 


24905 


0.24 


18780 


0.14 



Time of 

Rotation on 

Axis. 



d. h. m. 

25 7 48 

24 05 

23 21 

24 00 
24 37 

9 56 

10 29 

9 30? 

unknown. 



Hourly Mo- 
tion in Orbit 
in miles. 



109,000 
80,000 
68,000 
55,000 
30,000 
22,000 
15,000 
10,000 



Mean Dist., 

Earth 
as Unity. 



0.3870981 
0.7233316 
1.0000000 
1.5236923 
5.2027760 
9.5387861 
19.1823900 
30.0368000 



Sun 

Mercury. 
Venus ... 
Earth.... 

Mars 

Jupiter .. 
Saturn ... 
Uranus . . 
Neptune , 



Sidereal 

Revolution in 

Solar Days. 



87.9692580 

224.7007869 

365.2563612 

686.9796458 

4332.5848212 

10759.2198174 

30686.8208296 

60126.7100000 



Longitude 
of Perihelion. 



74 
128 

99 
332 

11 

89 
167 

47 



21 46.9 

43 53.1 

30 5.0 
23 56.6 

08 34.6 

09 29.8 

31 16.1 
12 56.7 



Longitude 

of Ascending 

Node. 



57 30.9 

54 12.9 



48 
98 

111 
72 

130 



03.5 

26 18.9 

56 37.4 

59 35.3 

8 11.0 



Eccentricity 

in parts 
of semi-axis. 



0.2055149 
0.0068607 
0.0167836 
0.0933070 
0.0481621 
0.0561505 
0.0466794 
0.0087195 



True 
Diameter 
in miles. 



888, 

3, 

7, 

7, 

4, 

88, 

79, 

34, 

41, 



Mean 
Apparent 
Diameter. 



32 02.9 
00 06.9 
00 16.9 

00 06.29 
00 36.74 
00 16.20 
00 04.0 
00 02.5 



The table of Asteroids will be found in Table II. 



470 



TABLES. 





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TABLES. 



471 



TABLE III.— Elements of the Moon. 

Mean distance from the Earth, (in terrestrial radii) 59.96435 

Mean sidereal revolution, (in mean solar days) 27.321661418 

Mean synodical revolution, " " " " 29,530588715 

Mean revolution of apogee, " " " " 3232.575343 

Mean nodical revolution, " « " " 6793.39108 

Mean longitude of-node, (Epoch, Jan. 1, 1801) 13° 53' 17".7 

Mean longitude of perigee, " '« " 266° 10' 7". 5 

Mean longitude of Moon, " " " 118° 17' 8".3 

Mean inclination of orbit 5° 8' 47".9 

Inclination of axis to orbit 1° 32' 9" 

Eccentricity of orbit 0.0548442 

Diameter in miles 2153 

Apparent diameter at mean distance from the Earth 31' 07" 

Apparent diameter at least distance from the Earth 33' 31". 07 

Apparent diameter at greatest distance from the Earth 29' 21".91 

Volume, (that of the Earth being 1) 0.0204 

Mass, " " " " , 0.011399 

Density, '< " " " 0.5657 

angular velocity per day 13.1764° 



TABLE IV.- 



-Altitude of the Principal Lunar Mountains, in English feet, calculated 
from the observations of Prof. Madler, by Hind. 



Name of Mountain. 



Newton 

Curtius 

Casatus 

Posidonius 

Short 

Moretus 

Calippus 

Mutus 

Huyghens 

Clavius 

Blancanus 

Kircher 

Hainzel 

Catharina 

Theophilus 

Tycho, (W. border) 

Picard 

Pythagoras 

Werner 

Macrobius 



Altitude in feet. 



23,800 
22,200 
20,800 
19,800 
18,700 
18,400 
18,300 
18,300 
18,000 
18,000 
18,000 
17,600 
17,500 
17,400 
17,300 
17,100 
17,000 
16,900 
16,600 
16,200 



Sblenographic Position. 



Longitude. 



16° E. 

3° W. 
35° E. 
29° W. 
10° E. 

7°E. 
10° W. 
30° W. 

2°E. 
15° E. 
21° E. 
43° E. 
32° E. 
23° W. 
26° W. 
12° E. 
64° W. 
60° E. 

3° W. 
45° W. 




TABLE V. — Diameters of some of the Principal Craters or Cavities on the Moon's 
surface, reduced from observations of Prof. Madler, by Hind. 



Bailly 

Clavius.. 
Schikard 
Ptolemy 

Schiller.. 
Phocyli.l( 



Breadth in English 
miles. 



149 
143 
127 
115 
113 
96 



Sklknographic Position. 



Longitude. 



65° E. 

15° E. 

55° E. 

3°E. 

38° E. 
56° E. 



Latitude. 



65° S. 
58° S. 
44° S. 
9°S. 
52° S. 



472 



TABLES. 



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TABLES. 



4T3 



TABLE VIII.— Dimensions of Saturn's Kings. 





Equatorial semi- 
diameters. 


English miles. 




4.4575 

3.9232 

3.8326 

2.9648 

0.26715 

0.43391 

0.42838 
1.48238 


172,130 
151,500 
148,000 
114,480 
10,316 
16,755 

18,628 
57,243 


Inner diameter of outer rino* 




Inner diameter of inner ring 




Breadth of inner ring 


Distance of inner ring (interior edge) from 

Saturn's limb 







TABLE IX.— Elements of Satellites of TTranus. 



Xn. of 
Sat. 


Sidereal Revolution. 


Mean Distance 
in miles. 


Mean Apparent 
Distance. 


Daily Motion. 


' 1 

2 
3 
4 
5 
6 


d. h. m. s. 

2 12 29 20 

4 3 28 8 

10 23 2 47 

13 11 6 55.21 

38 1 48 

107 16 39 56 


119,994 
170,863 
278,627 
379,921 
796,372 
1,592,640 


II 

13.54 
19.28 
31.44 

42.87 


II 

142.8373 
86.8732 
41.35133 
26.73943 



TABLE X.— Elements of Satellite of Neptune. 

Sidereal revolution 5d. 21h. Om. 17s. 

Apparent mean distance 16.75". 

Mean distance 232,000 miles. 

Inclination of orbit 29*^. 



474 



TABLES. 



TABLE XI. 

The following table shows the Eight Ascension, Declination, and North Polar 
Distance, also the Principal Star, of each Constellation which will be on the 
Meridian, in any latitude, in the evening of each month in the year. 

JANUARY. 



Reticulus 

Eridanus 

Taurus 

Camelopardalus 

Sceptrum Brandenburgiu 

Cela Sculptoria 

Auriga 

Dorado 

Orion 

Lepus 

Mens Mensas 

Equuleus Pictorius 

Columba , 



Eight 


iscension. 


Declination. 


h. m. 





o 


3 44 


56 


62 OS. 


4 00 


60 


10 S. 


4 20 


65 


16 ON. 


4 30 


67 30 


70 ON. 


4 30 


67 30 


15 S. 


4 44 


71 


40 S. 


5 00 


75 


45 ON. 


5 00 


75 


62 S. 


5 20 


80 





5 20 


80 


18 OS. 


5 20 


80 


75 S. 


5 30 


82 30 


52 S. 


5 40 


85 


35 S. 



N. Polar Dirt. J 


o 




152 





100 





74 1 


20 





105 





130 





45 





152 





90 





108 





165 





172 





125 






Principal Star. 



Acbernar. 
Aldebaran. 



Capella. 

Betelgeuse. 
Arneb. 



Phaet. 



FEBRUARY. 



Canis Major 

Officina Typograpbia... 
Telescopium Herscbelii 

Argo Navis (carina) 

Gemini 

Monoceros 

Canis Minor 

Lynx 

Argo Navis 



6 20 


95 


22 S. 


6 43 


100 45 


17 S. 


6 48 


102 


40 N. 


7 


105 


52 S. 


7 20 


110 


25 N. 


7 20 


110 





7 22 


110 30 


7 ON. 


7 24 


111 


50 N. 


7 44 


116 


50 S. 



112 
107 

50 
142 

65 

90 

83 

40 
140 



Sirins. 



Canopus. 
Castor. 

Proeyon. 



MARCH. 



Pisces Volans., 

Cancer 

Pyxis Nautica 
Felis , 



8 22 


125 30 


68 S. 


8 24 


126 


20 ON. 


8 50 


1,32 30 


30 S. 


9 40 


145 


20 S. 



158 

70 

120 

110 



Acubens. 



APRIL. 



Sextans 

Antlia Pneumatica 

Robur Caroli , 

Leo Minor 

Leo , 

Ursa Major 

Hydra 

Chameleon , 

Crater 



10 


150 





10 


150 


35 S. 


10 8 


152 


60 OS. 


10 12 


153 


35 ON. 


10 20 


155 


15 ON. 


10 30 


157 30 


60 ON. 


10 40 


160 


13 S. 


11 


165 


78 S. 


11 8 


167 


15 S. 



90 

125 

150 

55 

75 

30 

103 

168 

105 



Miaplaoidus. 

Regulus. 

Benettiasch. 

Alphare. 

Alkes. 



TABLES. 



475 



TABLE XI.— Continued. 
MAY. 



Crux 

Musca Australis 

Corvus 

Coma Berenices 

Cor Caroli 

Canes Venatici 

Virgo 

Centaurus 

Avis Solitarius 

Bootes 

Mons Menalus 

Circinus 

Libra 

Lupus 

Corona Borealis 

Quadrans Muralis 

Ursa Minor 

Triangulum Australis 

Norma 

Scorpio 

Apus 

Hercules 

Serpens 

Ara 

Ophiuchus 

Draco 

Cerberus 

Taurus Poniatowski.. 

Telescopium 

Scutum Sobieski 

CoronU Australis 

Lyra 

Sagittarius..- 

Antinous 

Pavo 

Aquila 

Sagitta 

Vulpecula 

Cygnus... 

Delphinus 

Capricornus 

Microscopium 

Octans 

Equuleus 



Right Aseeaaion. 


DeoUnation. 


h. m. 


o 


O ' 


12 20 


185 


60 S. 


12 20 


185 


70 OS. 


12 21 


185 4 


20 S. 


12 30 


187 30 


27 ON. 


12 48 


192 


40 N. 


13 12 


198 


40 N. 


13 14 


198 8 


SON. 


13 20 


200 


50 S. 



N. Polar Dist. 1 


o 




150 





160 





110 





63 





50 





50 





85 





140 






Acrux. 
Algorab. 
Cor Caroli. 
Spica. 



JUNE. 



14 


210 


20 S. 


14 28 


217 


30 N. 


14 40 


220 


15 ON. 


15 


225 


60 S. 


15 4 


226 


8 0S. 


15 20 


230 


45 S. 


15 25 


231 15 


28 ON. 


15 28 


232 


53 N. 


15 30 


232 15 


75 ON. 



110 
60 
75 

150 
98 

135 
62 
37 
15 



Arcturus. 

Zubeneschamali, 

Alphecca. 

Alrucabba. 



JULY. 



16 


240 


65 OS. 


16 8 


242 


50 OS. 


16 19 


244 45 


26 S. 


16 44 


251 


73 S. 


16 48 


252 


30 ON. 


17 


255 


3 ON. 


17 


255 


55 S. 


17 8 


257 






155 

140 

116 

163 

60 

87 

145 

90 



Antares. 
Eas Algetbi. 

Ras Alhague. 



AUGUST. 



18 


270 


65 ON. 


18 


270 


22 N. 


18 20 


275 


7 ON. 


18 20 


275 


52 S. 


18 32 


278 


10 S. 


18 40 


280 


40 S. 


18 44 


281 


38 N. 


19 


285 


30 S. 



25 

68 

83 

142 

100 

130 

52 

120 



Thuban. 



Wega. 
Ruchbah. 



SEPTEMBER. 



19 28 


292 





19 40 


295 


68 S. 


19 40 


295 


10 ON. 


19 48 


297 


18 ON. 


20 


300 


23 ON. 


20 26 


306 30 


42 ON. 


20 32 


308 


12 ON. 


20 40 


310 


20 S. 


20 40 


310 


38 S. 


21 


315 


85 S. 


21 4 


316 


5 ON. 



90 

158 

80 

72 

67 

48 

78 

110 

128 

175 

Sb 



Altair. 

Arided. 
Giedi. 



476 



TABLES. 



TABLE XI.— Continued. 
OCTOBER. 



Globus ^thereus . 

Indus 

Cepheus 

Grus 

Aquarius 

Pisces Australis... 

Lacerta 

Gloria Fredericii.., 
Pegasus , 

Toucan 

Pisces 

Cassiopeia 

Phoenix , 

Andromeda 

Maehina Electrica, 



Eight Ascension. 


Declination. 


h. m. 


o 


o ' 


21 20 


320 


30 S. 


21 20 


.320 


60 S. 


22 


330 


70 ON. 


22 


330 


45 S. 


22 20 


335 


14 S. 


22 20 


335 


30 S. 


22 28 


337 


45 ON. 


23 


345 


52 ON. 


23 


345 


15 N. 1 



120 

150 

20 

135 

104 

120 

45 

38 

75 



Principal Star. 



Alderamin. 
Fomalhaut. 
Markab. 



NOVEMBER. 



23 55 


358 45 


66 S. 


20 


5 


10 ON. 


40 


10 


60 ON. 


40 


10 


50 S. 


56 


14 


32 N. 


1 24 


21 


30 OS. 



156 
80 
40 

140 
58 

120 



El Rischa. 
Schedir. 

Alpheratz. 



DECEMBER. 



Triangulum Minus 

Cetus 

Triangulum 

Hydrus 

Aries 

Tarandus 

Musea Borealis 

Horologium 

Caput Medusa 

Fornax Cbemica..., 

Perseus , 

Solarium 



1 28 


22 


27 ON. 


1 40 


25 


12 S. 


1 48 


27 


32 N. 


1 52 


28 


66 S. 


2 20 


35 


22 N. 


2 32 


38 


80 N. 


2 40 


40 


28 ON. 


2 40 


40 


55 S. 


2 52 


43 


38 ON. 


3 


45 


30 S. 


3 12 


48 


48 ON. 


3 32 


53 


58 OS. 



63 
102 

58 
156 

68 

10 

62 
145 

52 
120 

42 
148 



Menkar. 

Arietis. 



Alffenib. 



TABLES. 



477 



TABLE XII. 

Containing the Magnitudes, Eight Ascensions, and Declinations, of the 
Principal Stars. 



/? Cassiopeia 

a Andromeda 

y Pegasi 

a Phenicis 

a Hydrge 

a Cassiopeia 

/JCeti 

y Cassiopeia 

/? Andromeda 

a Ursae Minoris... 

£ Eridani 

Arietis 

a Arietis 

Ceti 

a Ceti 

/? Persei 

a Persei 

i n Tauri 

j a Tauri 

a Auriga 

/? Orionis 

/? Tauri 

y Orionis 

6 Orionis 

a Columbse 

a Orionis 

a Argus 

a Canis Majoris... 

£ Canis Majoris .. 

6 Geminorum 

a G-eminorum 

a Canis Minoris.. 

/? Geminorum 

6 Argus 

a Hydras 

a Leonis 

y Leonis 

ri Argus 

a Crateris 

(i Ursae Majoris.. 

a Ursae Majoris... 

6 Leonis , 

/? Leonis 

/) Virginis 

y Ursae Majoris..., 

i Ursae Majoris... 

a Crucis 

6 Corvi , 

/? Corvi 

y Virginis 

6 Virginis 

£ Virginis 

a Virginis 

5 Ursae Majoris... 

r? Ursae Majoris..., 

n Bootis , 



Oommon Name. 



Caph 

Alpheratz. 
Algenib... 



Sohedir 
Diphda. 



Merach .... 

Polaris 

Achernar., 
Sheratan... 

Arietis 

Mira 

Menkar .... 

Algol 

Mirphak... 

Alcyone 

Aldebaran. 

Capella 

Rigel 

ElNath.... 
Bellatrix... 
Mintaka... 

Phad 

Betelgeuse, 
Canopus.... 

Sirius 

Adara 

Wasat 

Castor 

Procyon.... 
Pollux 



Alphard. 
Regains. 
Algieba . 



Alkes 

Merak 

Dubhe 

Zosma 

Denebola. 
Zavijava.. 
Phecda. ... 
Megrez.... 

Acrux 

Algorab... 



Vindemiatrix. 

Spica 

Mizar 

Benetnasch.... 
Muphrid 



Mag. 



2 

2 

3 

3 

3 
var. 

2 

3 

2 

2 

1 

3 

2 
var. 

2 
var. 

2 

3 

1 

1 

1 

2 

2 

2 

2 
var. 

1 

1 

2 

3 

2 

1 

2 

2 

2 

1 

2 

2 

3 

2 

2 

2 

2 

3 

3 

3 

1 

3 

2 

4 

3 

3 

1 

3 



Right Ascension. 



S. 
42 

53.9 

46.3 

1 

3.6 
32 18.3 
36 18.4 
47 5 
47 
6 30.3 
1 32 18.4 
1 45 49 



59 0.4 
11 16 
54 42 
57 46 
13 59.5 
38 52 
27 36.2 
5 59 
7 34.2 
17 7.7 
16 33 
24 36 
34 24 
47 19.3 
6 20 44 
6 38 46 



52 56 
11 27.6 
25 20.5 
31 42.5 
36 26.2 
40 7 
20 27.6 
38.7 
10 11 8 
10 39 26.7 
10 52 
10 62 8 

10 54 44.6 

11 6 23.4 
11 41 39.5 
11 42 22 

11 43 11 

12 7 28 
12 18 33.8 
12 21 35 
12 26 46.5 
12 33 33 
12 47 33 

12 54 13 

13 17 33.4 
13 17 35.2 
13 41 49.3 
13 47 46.8 



58 16 
28 17 

14 22 
43 12 
78 4 
55 44 

18 47 

59 50 
34 46 
88 32 

57 58 
20 1 

22 46 
3 42 

3 31 
40 20 

49 20 

23 39 
16 12 
45 50 

8 22 
28 28 

6 12 
24 

34 9 

7 22 
52 37 

16 31 
28 46 
22 14 
32 12 

5 35 
28 22 
54 5 

8 1 
12 40 

20 39 

58 55 

17 26 
57 14 
62 31 

21 19 

15 22 
2 40 

54 30 
57 55 
62 17 
15 37 

22 35 
34 

4 16 
11 49 
10 24 

55 45 

50 2 

19 7 



18 N. 

23 N. 
38 N. 
20 S. 
18 S. 
29 N. 

S. 

48 N. 
18 N. 
UN. 
28 S. 

24 N. 
28 N. 
18 S. 

4N. 

ON. 
27 N. 
UN. 

49 N. 
42 N. 

23 S. 
48 N. 

N. 

38 S. 
13 S. 

33 N. 

5 S. 
15 S. 

41 S. 

42 N. 
6N. 

36 N. 

20 N. 

43 S. 
57 S. 
26 N. 

N. 

21 S. 
54 S. 
18 N. 
57 N. 

2N. 
57 N. 

ON. 

3N. 
18 N. 

39 S. 

24 S. 

40 S. 
IS S. 

6 N. 
18 N. 
11 S. 
48 N. 
IS X. 

34 N. 



478 



TABLES. 



TABLE XII.— Continued. 



P Centauri 

o Draconis 

a Bootis 

a2 Centauri 

£ Bootis 

a2 Libra 

/? Ursae Minoris 

Bootis 

/? Librae 

a Coronae Borealis.... 

a Serpentis 

,51 Scorpii 

a Scorpii 

a Herculis 

Draconis 

y Draconis 

a Lyrae 

Lyrae 

y Lyrae 

(, Aquilae 

6 Aquilae 

/?Cygni 

y Aquilae 

a Aquilae 

Aquilae 

a Capricornii 

,(?2 Capricornii 

a Pavoni? 

a Delphini 

a Cygni 

611 Cygni 

C Cygni 

a Cephei 

Aquarii , 

Cephei 

£ Pegasi 

a Aquarii 

a Grus 

? Pegasi. , 

a Pisces Australis 

Pegasi 

a Pegasi 

y Cephei 



Common Name. 



Agena..., 
Thuban.. 
A re turns. 
Bungula., 
Mirae 



Kochab. 
Nekkar , 



Alphecca 

Unuk-al-Hay.. .. 

GraflBas 

Antares 

Ras-Algethi 

Ras-Alhague .... 

Etamin 

Wega 

Sheliak 

Sulaphat 

Deneb-el-Okab. 



Albireo.. 
Tarazed. 
Altair.... 
Alshain. 
Giedi.... 
Dabih.... 



Svalocin , 
Arided... 



Alderamin. 



Alpbirk 

Enif. 

Sadalmelik. 



Fomalhaut. 

Scheat 

Markab 

Errai 



Mag. 



1 

1 

3 

2 

2 

3 

2 

2 

2 

2 

1 
var. 

3 

2 

1 
var. 

3 



Bight 



h. m. s. 

13 53 37.9 

14 3 
14 9 2.8 
14 29 47.8 
14 38 39.2 
14 42 51.8 
14 51 10.7 

14 55 55 

15 9 12.4 
15 28 32.9 
15 37 7.6 

15 57 0.6 

16 20 31.3 

17 8 2 
17 27 9.4 

17 52 53 

18 32 L7 
18 44 43.5 
18 62 57 

18 58 44.6 

19 18 11.1 
19 24 16 
19 39 2L9 
19 43 42.4 

19 48 11.2 

20 10 0.3 
20 12 1 
20 14 9.1 
20 32 12 

20 36 29.3 

21 23.8 
21 6 45.9 
21 15 6.9 
21 23 55.3 
21 26 46.3 
21 37 3.8 
21 58 20 

21 59 4.4 

22 34 13.7 
22 49 37.6 
22 56 1 

22 57 32.4 

23 33 35.9 



59 40 


14 S. 


65 


8 


24 N. 


19 


56 


21 N. 


60 


13 


53 S. 


27 41 


15 N. 


15 26 11 S. 


74 44 53 N. 


41 


1 30 N. 


8 50 41 S. 


27 12 19 N. 


6 


53 


6N. 


19 24 17 S. 


26 


6 21s. 


14 


34 30 N. 


52 


24 37 N. 


51 


30 


36 N. 


38 


39 


5N. 


33 


11 


49 N. 


32 28 


30 N. 


13 


39 


5N. 


2 


49 45 N. 


27 37 42 N. 


10 


15 


47 N. 


8 


29 


19 N. 


6 


2 


62 N. 


12 


59 27 S. 


15 


16 


54 S. 


57 


11 


40 S. 


15 


21 


6N. 


44 


45 


51 N. 


38 


2 


19 N. 


29 


38 


3N. 


61 


58 20 N. 


6 


12 


24 S. 


69 


55 


29 N. 


9 


12 


44 N. 


1 


1 


21s. 


47 39 


37 S. 


10 


4 33 N. 


30 23 


23 S. 


27 


13 


ON. 


14 25 34 N. 


76 49 24 N. 



TABLES. 



479 



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480 



TABLES. 



TABLE Xiy. 

The Principal Observatories in the World, with their Latitudes and Longitudes. 



Xame. 


Latitude. 


Longitude. 




O / II 

67 8 67.8 N. 

60 26 57 K 

53 32 45.3 N. 

54 21 12.7 N. 

61 15 68 N. 

37 58 20 N. 

52 31 13.5 N. 

62 30 16.7 N. 
61 12 25 N. 

53 5 47 N. 
60 44 9.1 N. 

63 4 36 N. 

51 6 56 N. 

50 61 10.7 N. 

47 29 12.2 N. 

42 22 49 N. 

52 12 61.8 N. 
33 56 3 S. 

59 54 42.4 N. 

39 6 54 N. 
65 40 53 N. 

60 3 50 N. 

54 21 18 N. 
58 22 47.1 X. 

53 23 13 N. 

54 46 6.2 N. 

55 67 23.2 N. 

43 46 41.4 K 
46 11 69.4 N. 

38 64 26.1 N. 

51 31 48 N. 

50 56 5 N. 

51 28 38.2 N. 

53 33 5 N. 

61 48 36 N. 
41 14 42.6 N. 
55 47 23.1 N. 
51 30 12.7 N. 

54 42 50 N. 

48 3 24 N. 

51 20 20.1 N. 

52 9 28.2 N. 

53 24 47.8 K 
51 31 29.9 N. 
13 4 9.2 N. 

55 34 45 N. 

49 29 14 N. 

54 10 36 N. 

43 17 50.1 N. 

45 28 1 N. 

44 38 53 N. 

55 45 19.8 N. 
48 8 45 

40 51 46.6 N. 

46 58 20.6 N. 
51 45 36 N. 


h. m. s. 

8 22.78 W. 

1 29 8.8 E. ■ 
39 46.6 E. 

26 35.5 W. 

1 10.1 W. 

1 25 34.23 E. 
53 35.5 E. 
53 35.5 E. 
27 4.93 E. 
31 40.9 W. 
28 27 E. 

35 15.9 E. 

1 8 10 E. 

17 29 E. 

1 16 12.7 E. 

4 44 32 W. 

23.54 E. 

1 13 55 E. 
42 53.9 E. 

5 37 58.86 W. 

50 19.8 E. 1 

1 19 51.1 E. 
1 14 45 E. 

1 46 55 E. 
25 22 W. 
6 18 W. 
12 43.6 W. 
45 3.6 E. 
24 37.7 E. 
5 14 32 W. 
39 46.5 E. 
42 56.4 E. 


39 64.1 E. 
3 24.33 W. 
5 2b 41.3 W. 

2 30 31.93 E. 

46.78 W. 

1 22 0.5 E. 
56 32.3 E. 
49 28.5 E. 
17 67.6 E. 
12 0.11 W. 
37.1 W. 

5 21 3.77 E. ; 
10 4 W. 1 
33 51.4 E. 
33 48.4 W. 
21 29 E. 
36 47.2 E. 
43 43.2 E. 

2 30 17 E. 
46 26.5 E. 
67 0.3 E. 
2 7 55.1 E. 
5 2.6 W. 


Abo 




Armao'h 


Ashurst 


Athens 


Berlin 


Berlin, (New Observatory) 


Bilk 


Birr Castle, {Rosse) 


Bonn .* 


Bremen 




Brussels 


Buda 


Cambrido-e, (United States) 




Cape of Good Hope 




Cincinnati 




Cracow 






Dublin 






Florence 


Geneva 




Gottino"en 


Gotha 




Hamburg 


Hartwell 




Kasan 




Koniffsbero" 




Leipsic 




Liverpool 


London 


Makers town 




Markree 


Marseilles 

Milan 






Munich 




Nicolaeff 


Oxford . 





TABLES. 
TABLE XIV.— Continued. 



481 



Padua 

Palermo 

Paramatta 

Paris 

Philadelphia ,. 

Portsmouth 

Prague 

Pulkova 

Regent's Park, (London) 

Rome 

San Eernando 

Santiago 

Senftenberg 

St. Helena 

St. Petersburg 

Speyer 

Starlield 

Stockholm 

Strasburg 

Turin 

Upsala 

Venice 

Verona 

Vienna 

Viviers 

Warsaw 

Washington, (National Observatory) 

Wateringbury 

Wilna 

Wrottesley Hall 



Latitude. 



45 24 

38 6 
33 48 

48 50 

39 57 
50 48 

50 5 
59 46 

51 31 
41 53 
36 27 
32 26 

50 5 
15 55 
59 56 

49 18 

53 25 
59 20 

46 34 
45 4 
59 51 
45 25 
45 26 
48 12 
44 29 

52 13 
38 53 

51 15 

54 41 

52 37 



2 N. 

44 N. 

49.8 S. 
13 N. 

7.5 N. 

3 N. 

18.5 N. 

18.6 N. 

29.9 N. 
52 N. 

45 N. 
24.8 S. 

10 N. 
26 S. 
31 N. 
55.2 N. 

3.5 N. 
31 N. 
40 N. 

6 N. 
50 N. 
49.5 N. 

N. 

35.5 N. 

11 N. 
5 N. 

38.6 N. 

12 N. 
N. 
2.3 N. 



Longitude. 



47 

53 

4 

9 



4 

57 

1 



49 

24 

42 

5 

22 

1 

33 

11 

12 

31 

30 

1 10 
49 

44 

1 5 

18 

1 24 
5 8 

1 

1 41 
8 



s. 

29.2 E. 
25.6 E. 
6.25 E. 
21.5 E. 
38.36 W. 



23.9 

41.9 

18.5 

37.1 

54.7 

49.1 

18.9 

50.5 

50 

15.8 

46.5 

47.34 W. 

14.8 E. 



0.8 
48.4 
34.8 
25.4 

0.1 
31.9 
44.8 

8.5 
14 



E. 
E. 
W. 

39.8 E. 

11.9 E. 
53.57 W. 



Note. — The longitudes in the above table are reckoned from the meridian of 
Greenwich. 



31 



INDEX. 



Aberration, 164, 169, 396. 

Clairault on, 171. 

discovered by Bradley, 89, 170. 

effect of, 170. 

illustration of, 171. 

maximum of, 171. 

oflight, 89. 
planets, 171. 
Absolute motion, 21. 
Achernar, 256, 259. 
Achromatic telescope, 269, 342. 
Adams, calculations of the place of Nep- 
tune by, 362. 
Aerolites, 196. 
Air, color of, 161. 

impenetrability of, 16. 

material, 15. 
Albategnius, observations of, 330. 
Alcor, 213. 
Alcyone, 202. 
Aldebaran, 201. 
Alderamin, 231. 

Alexander, Prof., on the origin of the aste- 
roids, 360. 
Algenib, 233, 238. 
Algol, 185. 

a variable star, 238. 
Algorab, 248. 
Alhazen explains refraction, 395. 

writings of, 331. 
Alioth, 213. 
Almagest, 329. 

Almamon studied the sciences, 330. 
Almanzor, the first Arabian astronomer, 330. 
Alpha Centauri, 248. 
Alphabet, Greek, 161. 
Alphard, 247. 
Alphecca, 221. 
Alpherats, 233, 234. 
Alphirk, 232. 

Alphonse X. founded an astronomical col- 
lege, 332. 
Alphonsine tables, 332, 381. * 
Alrucaba, 217. 
Alshain, 229. 
Altair, 229. 

Altitude and azimuth instrument, 285. 
of a star found, Prob. VI., 309. 

explained, 99. 

reckoned, how, 99. 



Amazon, tides in the river, 386. 
American observations of transit of Ve- 
nus, 388. 
Anaxagoras, his theory, 325. 
Anaximander pronounced the Earth a 

sphere, 324. 
Ancha, 211. 

Ancient constellations, 200. 
Andromeda, 234. 
Angle, 12, 27. 

of incidence, 264. 
reflection, 264. 

parallactic, 177, 179. 

right, 28. 
Angular measurement, 27, 349. 
Animalcule, 16. 
Annual equation, by whom discovered, 368. 

motion of the Earth, 100. 

parallax, 177, 178. 
Annular eclipse, 130. 

nebulae, 191. 
Ansae, 371. 
Antares, 207. 
Antinous, 228. 

Antipodes of any place found, Prob. IX.,291. 
Antlia Pneumatica, 247. 
Antoeci of any place found, Prob.VIIL, 290. 
Antolycus, a cavity on the Moon, 84. 
Aperture of a telescope, 268. 
Aphelion, 47. 

Apparatus Sculptoris, 253. 
Apparent motion, 21. 

of the heavenly bodies, 99, 162. 

position, 28. 

time defined, 111. 
Appearance in travelling in a forest, 173. 

of the stars, 184. 
Appenines, lunar mountains, 84. 
Appolonius introduced epicycles, 326. 
Apus, 252. 
Aquarius, 210. 
Aquila, 228. 
Ara, 251. 

Arabians, astronomy of, 330. 
Arabs built observatories, 331. 
Arago estimates the number of comets, 143. 

his theory of zodiacal light, 198. 
Archimedes discovered the centre of gra- 
vity, 347. 
Aroturus, 220. 

483 






484 



INDEX. 



Argo Navis, 244. 

Aries, 201. 

Arietis, 201. 

Aristarchus, measurements of Sun and 

Moon, 326. 
Aristillus, a lunar cavity, 84. 
Aristotle, theory of, 345. 
Armils of Ptolemy Euergetes, 328. 
Artificial horizon, 277. 
Arzachel, tables of, 331. 
Ascension, right, and declination given, 
to find a star on the globe, Prob. 
II., 307. 
of a star, Prob. I., 307. 
Asellus Australis, 204. 
Asellus Borealis, 204. 
Asterion, 219. 
Asteroid planet a disc, 360. 

supposed size of, 360. 
Asteroids, 56, 356. 

astronomical signs of, 56. 
distance from the Sun, 57. 
form of their orbits, 57. 
inclination of their orbits, 57. 
names of, <fcc., 56. 
opinions as to their origin, 360. 
period of their revolution, 57. 
smaller than the other planets, 57. 
Astrea discovered by Hencke, 357. 
Astrolabe, treatise on, 331. 
Astrology formerly studied, 320. 
Astronomers, Egyptian, 156. 
Astronomical clocks, 110. 

time indicated by, 279. 
tables of India, 321. 
year, 382. 
Astronomy of modern Europe, 332. 
the Arabians, 330. 
Chaldeans, 323. 
Chinese, 322. 
Egyptians, 322. 
Greeks, 324. 
Indians, 320. 
Persians, 331. 
Phoenicians, 324. 
Tartars, 331. 
physical, 15. 
sidereal, 158. 
systems of, 155. 
Atmosphere, 42. 

density of, 166. 
its effects, 43. 
of the Moon, 78. 
Sun, 349. 
Atoms defined, 345. 

Attraction of Sun and Moon compared, 120. 
Auriga, 239. 
Avis Solitarius, 250. 
Axis, declination, of a telescope, 270. 
of a lens, 262. 

the Earth inclined, 104. 
heavens, 12. 
Azimuth explained, 98. 
how reckoned, 98. 



Babylonic hours, 379. 

Bacon, Roger, explained refraction, 395. 

suggestions of, 332. 
Baily, observations on the Moon, 368. 
Baiton Kaitos, 255. 

Ball discovered the division of Saturn's 
ring, 372. 

time, at Greenwich, 111. 
Barker, theory of, 184. 
Bartschius, 239. 

constellation formed by, 237. 
Bay of Fundy, tides in, 123. 
Bayer, description of constellations by, 337. 

introduced Apus, 252. 
Chameleon, 251. 
Indus, 255. 
Beam of light, 261. 
Beehive, 204. 
Bellatrix, 241. 

Belli, Prof., opinions of the heat of moon- 
light, 365. 
Benetnasch, 213. 

Bentley, Dr., Newton's letter to, 347. 
Bessel, observations of, 183. 

supposed the existence of Neptune, 362. 
Betelgeuse, 241. 
Biela's comet, 142. 

appearance of, 153. 

orbit of, 153. 

period of, 153. 
Binary systems, 187. 
Biot, his theory of zodiacal light, 198. 
Birs-i-NimrM, mound described, 323. 
Bissextile, why so called, 113. 
Bode, 257. 

introduced Telescopium, 239. 
Bode's law, 356, 360. 

table of distances of planets, 361. 
Bond, G. P., illustration by, 192. 

observations on Saturn's ring, 372. 
Bootes, 220. 

Borelli, theory of the orbits of comets, 390. 
Borosus, hemisphere of, 327. 
Bouvard, his observations of Uranus, 361. 
Bradley confirmed Roemer's theory, 89. 

discoveries of, 89, 170, 224, 277, 340, 
396. 

observations of, on Etanin, 224. 
Brahe discovered annual equation, 368. 

variation, 368. 
Brahmins, calculations of, 321. 
Brorsen's comet, 142. 
Bulk of a sphere, how found, 68. 
Bulks of Earth and Moon, 67. 
Bull's eye, 202. 

CiESAR, Julius, remodelled the ealendar,327. 
Calendar, 380. 

remodelled by Caesar, 327. 
Calippic period, 114, 326. 
Calippus, cycle of, 326. 
Camelopardalus, 239. 
Cancer, 204. 
Canes Venatici, 218. 



INDEX. 



485 



Canicular period, 322. 
Canis Major, 243. 
Canis Minor, 242. 
Canopus, 244. 
Capella. 239. 
Caph, 235. 
Capricornus, 210. 
Caput Medusa, 237. 
Cardinal points of horizon, 98. 
Carina, 244. 

Cassegrainian telescope, 272. 
Cassini discovered two rings around Sa- 
turn, 372. 

erected a gnomon at Bologna, 341. 

first astronomer royal of Paris, 337. 

observations of the Moon by, 368. 
on Saturn's ring by, 372. 

opinions of the theory of gravita- 
tion, 347. 
Cassiopeia, 234. 

new star in, 402. 
Castor, 203. 

Catalogues of stars, 161, 186. 
Cela Sculptoria, 244, 257. 
Celestial poles, 162. 

sphere, 11. 
Central eclipse, 128. 
Centre of gravity, 19, 347. 

of solar system, 20. 
Centrifugal force, 26, 45, 48, 96, 348. 
Centripetal force, 26, 49, 96. 
Centauri, Alpha, 177. 
Centaurus, 248. 
Cepheus, 231. 
Cerberus, 225. 

Ceres discovered by Piazzi, 343, 357. 
Cetus, 255. 

Chacornac discovered the twenty-fifth as- 
teroid, 358. 

thirty-third asteroid, 359. 
Chaldean astronomy, 323. 
Challis, observations of, 153, 362. 
Chameleon, 251. 
Chara, 219. 
Cheseaux, 146. 

Childrey first observed zodiacal light, 405. 
Chinese astronomy, 322. 
Chord, 14. 

Cinnabar, divisibility of, 345. 
Circinus, 250. 
Circle, divisions of, 28. 

mural, 281. 

transit, 284. 
Circumference of Earth's orbit, 51. 
Circumpolar stars, 213. 
Civil day, 111, 379. 

year, 381. 
Clairault, calculations by, 392. 

on aberration, 171. 
Classification of stars, 160. 
Cleomedes' method for measuring the dia- 
meter of the Sun, 329. 
Clepsydra described, 329. 

presented to Charlemagne, 330. 



Clocks, astronomical, 110. 

common, show mean solar time, 110. 

electric, 279. 

first used in observatories by Wal- 

ther, 335. 
of the Washington observatory, 280. 
transit, 279. 
Cloudy stratum around the Sun, 349. 
Cluster in Canes Venatici, 219. 
Capricornus, 210. 
Crucis, 250. 
Libra, 207. 
Lyra, 227. 
Pegasus, 233. 
Perseus, 238. 
Sagittarius, 209. 
Scorpio, 208. 
Toucana, 254. 
Clusters and nebulae, 190. 

form of, 191. 
Colebrooke, T. C, 321. 
Collimator, 278. 
CoUo Ceti, 255. 
Color of comet of 1811, 150. 

Sirius changed, 184. 
Colors of stars, 184. 
Columba, 243. 
Colure, equinoctial, 212. 
Colures, 115. 
Coma, 140, 145. 

Berenices, 220. 
Comet of 1844, period of, 149. 
orbit of Biela's, 153. 
seen by Cysatus, 153. 

Euphorus, 153. 
tail of, 140, 146. 
of 1843, 146. 

appearance of, 150. 
distance of, 151. 
nucleus of, 151. 
tail of, 151. 
velocity of, 151. 
of 1744, 146. 
1770, 154. 
1811, 145. 

color of, 150. 
period of, 149. 
of Halley, 151. 
Lexell, 154. 
1680, 147. 

distance at aphelion, 149. 
size of, 148. 
derivation of the word, 140. 
elements of, 151. 
head of, 140, 145. 
nucleus of, 140. 
of Biela, its discoverer, 153. 
period of, 153. 
Cometary nebula in Scorpio, 208. 
Comets, 30, 140. 

change of appearance of, 141. 
diameters of heads, 145. 
distinguished from planets, how, 140. 
form of, 144. 



486 



INDEX. 



Comets, heat of, 147. 

map of, 146. 

move in all directions, 144. 

nucleus of, 144, 145. 

number of, 143. 

of short period, 142. 

orbits of, 140. 

remarkable, 147. 

substance of, 144. 

tails of, 391. 

telescopic, 146, 390. 

time of their greatest splendor, 148. 

with hyperbolic orbits, 141, 143. 

visible in the day-time, 146. 
Comparative size of Earth and Jupiter, 59. 
Complementary colors of stars, 188. 
Compound motion, 22, 23. 

stars, 184, 187. 
Concave mirrors, 264. 
Concentric circles, 11. 
Conical shadows, 124. 
Conjugate axis, 13. 
Conjunction, 39. 

inferior, 39. 
Constellations, 198. 

ancient, 200. 

list of, 199. 

modern, 200. 

northern, 212, 213. 

origin of, 406. 

represented, how, 200. 
Convex lenses, 262. 
Convexo-concave lens, 262. 
Copernican system, 157. 
Copernicus, 157. 

a lunar crater, 86. 

theory of, 332. 
Cor Caroli, 219. 

double star, 220. 

Hydra, 247. 

Scorpii, 207. 
Corona, 129. 

Australis, 251. 

Borealis, 221. 
Corvus, 248. 
Crater, 247. 

terrestrial, 85. 
Crepusculum, 169. 
Cross in Cygnus, 230. 
Crux, 250. 
Curvilinear motion, 24. 

produced, how, 26. 
Cycle, Calippic, 326. 

defined, 114. 

Metonic, 114, 325. 

of the Moon, 114, 166, 382. 

of the Sun, 114, 382. 

Paschal, 382. 

Cycles, 382. 

use of, 114. 
Cygni, (star 61,) 231. 
Cygnus, 230. 
Cynosura, 217. 
Cysatus, account of a comet, by, 153. 



D'Arrest's comet, 142. 

Day and night on the Moon, 364. 

to find the longest in the frigid zone, 

Prob. XXV., 302. 
civil. 111. 
given, to find what other day is of the 

same length, Prob. XXII., 299. 
its length invariable, 100. 
kept, by difi"erent nations, 379. 
length of natural, 109, 110. 
light and darkness, cause of, 96. 
longest, at the poles, 107. 
natural, 109. 
on the Moon, 69. 
sidereal, 44, 109. 

length of, 109, 110. 
solar, 109. 

common reckoning of time, 110. 
Days and nights equal at the equator, 102. 
when, 107. 
length of, 102. 
length of at the poles, 107. 
long in summer, why, 105, 106. 
shorter in winter, 105. 
Declination, 13. 

axis of a telescope, 270. 
Degrees, 28. 

Delambre, opinion of, on Chaldean astro- 
nomy, 323. 
Delphinus, 232, 
Democritus, his opinion of the Milky 

Way, 325. 
Deneb, Cygni, 230. 

Kaitos, 256. 
Deneb-el-Okab, 229. 
Denebola, 217, 204. 
Density, 21. 

of the Earth, 68. 
Moon, 68. 
Descartes, his theory, 334. 
De Stella Martis, theory contained in, 346. 
De Vice's comet, 142. 

observations of Saturn's ring, 373. 
Diagonal eye-piece, 269. 
Diameter, fixed stars without apparent, 159. 
of Earth's orbit, 51. 
known, 176. 
Jupiter, 59. 

Moon as seen from the Earth, 364. 
nucleus of comet of 1843, 151. 
Saturn, 61. 
Uranus, 63. 
Diameters, comets, table of, 145. 

Earth and Moon compared, 364. 
heads of comets, 145. 
planets measured, 351. 
Digges, Leonard, 160. 
Digits, 128. 
Diodorus Siculus, account of a comet, 143. 

on Egyptian discoveries, 322. 
Dione, the fourth satellite of Saturn, 376. 
Dioptra described, 328. 
Dipper, 213. 
Direction of shadows, 124. 



INDEX. 



487 



Disc, stars have no measurable, 139. 
Discoveries on the Moon, 80. 
Discovery of Jupiter's satellites, 88. 
Dispersion of rays, 269. 
Distance between two places found, Prob. 
XL, 291. 

of comet of 1680 at aphelion, 149. 
stars, 177, 400. 
Sun, how ascertained, 137. 
Diurnal motion of the Earth, 96. 
Divisibility of matter, 345. 
Divisions of time, 109. 
Dollond, inventions of, 342. 

perfected the achromatic telescope, 270. 
Dome of an observatory, 267. 
Dorado, 257. 
Doradus, 30, 258. 

Dorfel, his theory of orbits of comets, 390. 
Double-concave lens, 262. 

convex lens, 262. 

nebulae, 193. 

in Virgo, 194. 

stars, 187. 
Draco, 224. 
Dubhe, 214. 
Du Champ, Indian tables of, 321. 

Earth, 42. 

absorbs heat, when, 105. 

annual motion of, 100. 

appearance of, to an observer on the 

Moon, 67, 363. 
a spheroid, 45. 
attracts Sun and Moon, 49. 
axis of, 44. 

circumference of its orbit, 51. 
density of, 68. 
diameter of, as seen from the Moon,364. 

compared with the Moon, 131. 

its orbit, 51. 
distance of, from the Sun, 31, 46. 
diurnal motion of, 96. 
effect of a more rapid rotation, 348. 
fixed in the lunar sky, 77. 
form of, 44, 353. 

path of, 47. 

shadow of, 131. 
furnishes a standard of weights and 

measures, 353. 
hourly motion of, 50, 162. 
in one of foci of Moon's orbit, 74. 
invisible from Neptune, 363- 
length of shadow of, 131. 
light, 67. 

mass, adapted to that of the Sun, 51. 
motion of, 47, 96. 
moves in a curved line, 50. 
orbit of, a curve, 50. 

elliptical, 107. 

used as a base line, 176. 
period of revolution of, 51. 
primary centre of Moon's motion, 363. 
radiates heat, 105. 
retained in its orbit, 48. 



Earth, revolves on its axis, 43. 
round the Sun, 46. 

rotation on its axis equable, 108. 

size of, 46. 

surface of, 43. 

velocity of, 50. 

greatest near the Sun, 108. 
in her orbit, 102. 
Ebb and flow of tides, 123. 
Ebn-Junis, observations of, 330. 
Eclipse, annular, 130. 

cause of lunar, 132. 
solar, 127. 

central, 128. 

duration of total, 129. 

partial, 128. 

solar, 125. 

visible over what extent, 129. 

to find when one will occur, Prob. 
XX., 319. 

total, 128. 
Eclipses, cause of, 123. 

defined, 123. 

kinds of, 125, 128. 

occur, when, 125. 

of Jupiter's satellites, 369. 
use of, to us, 371. 

recur regularly, 134. 
Ecliptic, 13, 104, 383. 

and zodiac, 114. 

defined, 114. 

inclination of, 115. 

situation of, 115. 
Egyptian astronomy, 322. 

day, 379. 

estimate of the Sun's diameter, 328. 

system, 156. 

year, 322. 
Elasticity of bodies, 17. 
El-Bakar, 195. 
Electric clock, 279. 

fluid, velocity of, 400. 
Electro-chronograph, 279. 
Elements of a comet, 151. 

comet's orbit, 141. 
Ell and Yard, 241. 
Ellipse, 13, 353. 

construction of, 354. 
EllNath, 202. 
Elongation, 38. 
El-Rakis, 224. 
El-Rischa, 212. 

Enceladus, the second satellite of Sa- 
turn, 376. 
Encke, Prof., his observations of Saturn's 

ring, 372. 
Encke's comet, 142. 
Enif, 233. 
Eavelop, 140. 

of comet of 1811, 150. 
Epact, 382. 

Epicycles introduced by Appolonius, 326. 
Equation of the centre explained, 366. 

time, 112. 



488 



INDEX. 



Equator defined, 114. 

different positions of, 103. 

or equinoctial, 11. 
Equatorial telescope, 270. 
Equilibrium of heat maintained, 108. 
Equinoctial armil, 328. 

colure, 115, 212. 

line defined, 115. 

points, 16-1. 
Equinox, vernal, 164. 
Equinoxes, 13, 115. 

precession of, 164. 

revolution of, 164. 
Equuleus, 233. 

Pictorius, 244. 
Eratosthenes, observations of, 326. 
Eridanus, 256. 

Errai will be the pole star, 232. 
Etanin, 224. 
Ethereal vortices, 334. 
Eudoxus built an observatory, 325. 
Euler, his designation of the centre of gra- 
vity, 347. 
Euphorus, his account of a comet, 153. 
Evection explained, 366. 
Exterior planets, 51, 52. 
External contact, 135. 
Eye-piece, diagonal, 269. 

negative, 269. 

of a telescope, 268. 

positive, 269. 

Fabricus discovered Mira, 255. 

Faculse, 33. 

Faye's comet, 142. 

Felis, 247. 

Ferguson, his discovery of the thirty-first 

asteroid, 359. 
Filar micrometer, 274. 
Fish's mouth, 242. 
Fixed stars, 158, 159. 

shine by their own light, 158. 

supposed to have motion, 158. 

their use, 159. 

transit, 270. 
Flamsteed, 339. 

observations of, on Uranus, 361. 
Fluid mass, form of, in a state of rest, 353. 
Fluids, motion of, 117. 
Focal point, 265. 
Fomalhaut, 253. 
Forbes, his experiments on the heat of 

moonlight, 364. 
Force, centrifugal, 26, 348. 

centripetal, 26, 49. 

projectile, 26. 
Form of the Earth, 353. 
Fornax Chemica, 258. 
Friction, 22. 
Full Moon, 71. 

G-ALAXY, 180. 
Galileo, discoveries of, 333. 
experiments of, 345. 



Galileo, his first view of Saturn, 371. 

made the first telescope, 341. 

observations of, 158. 

rendered blind, 337. 
Galle, Dr., first observed Neptune, 362. 

observations of Saturn's ring, 375. 
Gascoigne, his improvements, 341. 
Gasparis discovers the tenth, eleventh, 
thirteenth, fifteenth, sixteenth, twen- 
tieth, and twenty-fourth asteroids, 
358. 
Gassendi observed transit of Mercury, 136. 
Gemini, 202. 

Genghis Khan, his love of science, 331. 
Geocentric place of a planet, 115. 
Ghost, Ramsden's, 277. 
Globes, relative contents of two, 34. 

treatise on, 287. 
Globular nebulae, 191. 
Globus ^thereus, 253. 
Gnomon, invention and description of, 327. 
Gold, division of a single grain, 345. 
Golden number, 382. 

its origin, 325. 

yard, 241. 
Goldschmidt discovers the twenty-first as- 
teroid, 353. 

twenty-sixth asteroid, 359. 
Gomeisa, 243. 
Gould, B. A., Jr., his observations on the 

Moon, 369. 
Graduated circles, 281. 
Graham discovers Metis, 358. 
Gravitation, 17. 

decrease of, 17. 

force of, at the Moon, 18. 

progress of its discovery, 346. 

theory of, when confirmed, 347. 

universal, 117. 
Gravity, 49. 

its force, 18. 
Great Bear always above horizon, where, 

214. 
Grecians guided their ships by Ursa Ma- 
jor, 214. 
Greek alphabet, 161. 
Greeks, astronomy of, 324. 
Gregorian telescope, 271. 
Gregory, his telescope, 342. 
Greenwich observatory, erection of, 339. 
Grumium, 224. 
Grus, 254. 
Guards, 217. 

Hall, his discovery, 270. 
Halley, 339. 

constellation formed by, 246. 

his account of meteors, 196. 

theory of proper motion, 397. 

observes the transit of Mercury, 388. 

predicted a comet's period, 151. 
Halley's comet, 151, 391. 

in 1835, 152. 

the first predicted, 152. 



INDEX. 



489 



Halley's comet, period of, 151. 
Hanging level, 276. 

Hans Sloaae's observation of a meteor, 196. 
Harding discovers Juno, 343, 357. 
Harriot, his contributions to science, 337. 
Harun-al-Raschid cultivated the sciences, 

330. 
Harvest-moon, 79. 

Ulustrated, Prob. XXIX., 306. 
Heat absorbed by the Earth, when, 105. 

equally dispersed, 108. 

equilibrium of, maintained, 108. 

of comets, 140, 145, 147. 
moonlight, 364. 

radiated by the Earth, 105. 
Heavens, appearance of at Saturn, 93. 

for any given time, Prob.XII., 313. 

poles of, 162. 
Hebe discovered by Hencke, 357. 
Height of lunar mountains, 83. 

object estimated, how, 83. 

tidal wave, 120. 
Heliocentric place of a planet, 115. 
Hell, Maximilian, 256. 
Hemisphere, 11. 

Hencke discovers Astrea and Heb%^ 357. 
Hercules, 221. 
Herodotus, history by, 327. 
Herschel, Caroline, 340. 

astronomical labors of, 191. 

cluster discovered by, 236. 
Herschelian telescope, 271. 
Herschel, Sir J,, catalogue of stars by, 246. 
30 Doradus, 258. 

on the atmosphere of the Sun, 349. 

on the heat of moonlight, 364. 
Herschel, Sir W., 340. 

discovered Uranus, 63. 

great telescope of, 273. 

knowledge of the stars, 201. 

observations of, 191. 
on the Moon, 368. 

theory of constitution of the Sun, 350. 
Milky Way, 181. 
proper motion, 173, 397. 
Herschel's forty feet reflector, 272. 
Hesiod, his designation of the seasons, 324. 
Hevelius, constellations formed by, 221, 247. 

observations of, 337. 

observes Saturn, 372. 

transit of Mercury, 388. 
Hind, discoveries of, 358, 359. 

table of comets by, 143. 
Hipparchus acquainted Avith acceleration 
of falling bodies, 346. 

catalogue of stars made by, 200. 

discovered precession, 393. 

discoveries of, 326. 
History of astronomy, 320. 

of nineteenth century, 343. 
Honores Frederici, 232. 
Hooke, Robert, his discoveries in science, 

346. 
Horizon, 97. 



Horizon, artificial, 277. 

cardinal points of, 98. 

extent of, 163. 

rational, 97. 

sensible, 97. 
Horizontal collimator, 278. 

parallax, 136, 174. 
Horologium, 259. 

Horrox first saw transit of Venus, 137, 337. 
Horsley,meteorological observations of, 365. 
Houlagou Khan built an observatory, 331. 
Hour being given, to find what hour it is at 
any other place, Prob, VII., 292. 

of night found, Prob. X., 312. 
Hourly motion of Earth, 162. 
Hours, planetary, 379. 
Hueck, his observations, 160. 
Hull, 244. 

Humboldt, a lunar cavity, 84. 
Hunt, Robert, experiments of, 373. 
Hunter's moon, 79. 
Huyghens discovers Saturn's ring, 372. 

observations of, 334. 

opposed the theory of gravitation, 334. 
Hyades, 201. 
Hydra, 247. 
Hydrus, 259. 

Hypatia, a scientific writer, 330. 
Hyperbolic orbits of comets, 141, 143. 
Hyperbolically-moving comets, 390. 
Hyperion, the seventh satellite of Saturn, 
376. 

Illuminating power of light, 26]. 

Incidence, angle of, 264. 

Inclination of comet's orbit, 141, 142. 

Jupiter's axis, 60. 

Moon's orbit, 386. 
Indian astronomical tables, 321. 

astronomy, 320. 

zodiac, 321. 
Indus, 254. 
Inertia, 17. 

of the waters of the ocean, 118. 
Inferior conjunction, 39. 
Instrumental adjustments, 275. 
Interior planets, 34. 
Internal contact, 135. 
Irregularities of the Moon's motions, 78, 365 

Jacob's staff, 241. 

Jansen, his invention, 336. 

Japetus, the eighth satellite of Saturn, 375. 

Jewish day, 373. 

Job's Coffin, 232. 

Jones, Geo., observations on zodiacal light, 

405. 
Jones, Sir W., on the observations of the 
Chaldeans, 323. 

opinion of the Indian zodiac, 321. 
Julian calendar, 380. 

epoch, 380. 

year, 327. 
Juno discovered by Harding. 343, 357. 



490 



INDEX. 



Jupiter, 58. 

a spheroid, 59. 

brighter than Mars, 58. 

diameter of, 58, 59. 

distance from the Sun, 58. 

extent of his shadow, 369. 

inclination of his axis, 60. 

length of his day, 59. 

moons of, 60, 87. 

period of revolution of, 59. 

period of rotation, 59. 

telescopic appearance of, 60. 

quantity of light received by, 60. 
Jupiter's satellites, 60, 87. 

discovery of, 88. 

eclipses of, 89, 170, 369. 

exhibit phases, 89. 

farthest, 89. 

nearest, 89 

number of, 88. 

orbits of, 88. 

revolve on their axes, 89. 

Kater, Capt., invented the collimator, 278. 

observations on the Moon, 368. 
of Saturn's rings, 372. 
Keel, 244. 
Kepler, a lunar crater, 86. 

discovered the form of planetary or- 
bits, 355. 

discoveries of, 334. 

discovery of his third law, 336. 

estimates the number of comets, 143. 

observations of, 158. 

suspected the existence of an asteroid 
planet, 360. 
Kepler's laws, 354. 

theory of falling bodies, 346. 

the number of the planets, 360. 

works, 336. 
Kirch, 257. 
Kitalpha, 233. 
Kochab, 217. 
Korneforos, 222. 

Lacaille, 244. 

constellations formed by, 246, 251, 252. 

discoveries of, 340. 
LacedEemon, first sundial set there, 327. 
Lacerta, 232. 
Lady's Wand, 241. 
Lagging of the tides, 118. 
La Hire, his experiments, 364. 
Lakes, why not subject to tides, 123. 
Lalande, calculations by, 392. 

estimates the number of comets, 143. 

observed Neptune twice, 363. 
transit of Mercury, 388. 
La Loubcre, Indian tables of, 321. 
Landgrave of Hesse built an observatory, 

333. 
La Place, theory of constitution of the 
Sun, 350. 

Saturn's rings, 372. 



Lassell and Dawes' observations of Sa- 
turn's ring, 373. 
discovered Neptune's moon, 64. 
Latham, his description of refraction, 396. 
Latitude, 13. 

and longitude found, Prob. I., 287. 
given, to find the place, Prob. II., 

287. 
of a star, Prob. IIL, 308. 
of a place, 163. 

two places, diflference found, Prob, 

IIL, 288. 
those places having the same as a 
given place, Prob. VI., 289. 
Law of falling bodies, 18. 
Laws, Kepler's, 354. 
of motion, 21, 347. 

the heavenly bodies, 15, 
Layang, observations at, 322. 
Leap year explained, 113. 
rule for finding, 113. 
Le Gentil, Indian tables of, 321. 
Leibnitz, opposed the theory of gravita- 
tion, 347. 
Le Monnier, constellation formed by, 250. 
observed Uranus as a fixed star, 361. 
Lens, 262. 
Leo, 204. 

Minor, 217. 
nebula in, 205. 
Lepaute, Madame, her calculations, 392. 
Lepus, 243. 
Level, 276. 

varieties of, 276. 
Le Verrier indicated the situation of Nep- 
tune, 362. 
opinions of, on the origin of the as- 
teroids, 360. 
now called Neptune, 64. 
predicted the place of Neptune, 343. 
Lexell's comet, 154, 392. 

orbit of, 154. 
Libra, 206. 
Light, aberration of, 89. 

and heat derived from the Sun, 30. 
Bradley discovered aberration of, 89. 
from planetary nebulse, 193. 
of Milky Way, 181. 
Sirius, 161, 243. 
stars measured, 161. 
on the Moon, 87. 
particles of, minute, 16. 
produced, how, 260. 
progessive motion of, 170. 
propagation of, 261. 
properties of, 260. 
source of, 349. 
time required to reach us from the 

stars, 177. 
velocity of, 89, 170, 172, 179, 260, 400. 
zodiacal, 198. 
Lilienthal, observers assembled there, 357. 
Limb of an instrument, 275. 
Line of direction, 19. 



INDEX. 



491 



Lion's Heart, 204, 

List of constellations, 199. 

Little Dipper, 217. 

Locke, Dr., his clock, 280, 

Logarithms, invented by Napier, S3t. 

Longitude, 13. 

of a heavenly body, 13. 
perihelion, 141, 142. 
the node, 141, 142. 
two places, difference found, Prob. 
IV., 288. 
reckoned, 164. 

to find those places having the same 
as a given place, Prob. VII., 290, 
Longomontanus, theory of, 157. 
Loomis on the heat of moonlight, 365. 
Lumiere cendree, 364. 
Lunar cavities, diameter of, 84. 

eclipse, to find where it will be visible, 

Prob. XXVIIL, 304* 
eclipses, 131. 

cause of, 132. 
duration of, 132. 
mountains, appearance of, 83. 

height of, 83. 
plains, 86. 
shadows, decrease of, 84. 

interesting to observers, 83. 
Luther, discoveries by, 358, 359. 
Lupus, 250. 
Lynx, 239. 
Lyra, 225, 

Machina Electrica, 259. 
Maclear, his observations, 145. 
Macrobius on the knowledge of the Egyp- 
tians, 322. 
Maculge, 33. 
Magellanic clouds, 194. 
Magnitude, 15. 

of the stars, 160, 182. 
planetary nebulae, 193. 
Malcolm, Sir John, discoveries of, 335. 
Manilius, a lunar mountain, 84. 
Maps of stars, 161. 

Maraldi opposed the theory of gravita- 
tion, 347. 
Mare Crisium, a lunar plain, described, 86. 

Humorum, a lunar plain, 86, 
Markab, 233. 
Marth, discovery by, 359. 
Mars, 53. 

appears brightest, when, 54. 

apparent size of, 54. 

axis of, inclined to his orbit, 55. 

distance of, from the Earth, 53. 
Sun, 53. 

in opposition to the Sun, 54, 

length of his day, 55. 

on the meridian at midnight, 53. 

orbit exterior to that of the Earth, 53. 

period of, 53. 

real size of, 54. 

seen through a telescope, 55. 



Mars, snow at his poles, 55. 

sometimes gibbous, 54. 

year of, 53. 
Maskeline, his letter to Thomas Penn, 389. 

observations on the Moon, 369. 
Mass, 49. 

of the comet of 1770, 154. 

of the Earth, adapted to that of the 
Sun, 5L 
Matter, 15. 

compressibility of, 16. 

dilatability of, 17, 

divisibility of, 16, 345. 

general properties of, 15. 

impenetrability of, 16. 
Maury, Lieut,, observations, 153. 
Mean time defined. 111. 

reckoned, how. 111. 
Measurement, angular, 349. 
Measure of time, 108. 
Megale Syntaxis, 329. 
Megretz, 214, 217. 

Melloni, his experiments on moonlight, 365. 
Meniscus lens, 262, 
Menkab, 255, 
Menkalinan, 239. 
Menkar, 255, 
Merak, 214. 
Mercury, 35. 

diameter of, 35. 

distance of, 35. 

heat on, 37. 

nodes of, 136. 

revolution of, 35. 

rotation of, 36. 

transit of, 136, 387. 
Meridian, 99. 

line found by the Egyptians, 322. 

mark, 267, 
Meridians, 11. 

Messier, his theory of Saturn's ring, 372. 
Meteoric stones, great falls of, 403. 
Meteorite, analysis of, 404. 
Meteorites, 196. 

and aerolites, 403. 

of foreign origin, 197. 
Meteors, 195. 

direction of, 196. 

origin of, 195. 
Metonic cycle, 114, 325, 382. 
Miaplacidus, 245. 
Micrometer, 178, 274, 

illuminated, how, 275. 

spider line, 283. 
Microscopes, reading, 281. 
Microscopium, 252. 
Mid-day, 97. 
Midnight, 97. 
Milk-dipper, 209. 
Milky Way, ISO, 401. 

form of, 181. 

light of, 181. 

situation of, 181. 
Mimas, the first satellite of Saturn, 376. 



492 



INDEX. 



Mira, 184, 255. 

Mirach, 221, 234. 

Mirfak, 238. 

Mirrors or speculae, 264. 

Mitchell, Miss, received a gold medal, 390. 

Mizar, 213. 

Modern constellations, 200. 

Molyneux and Bradley, observations of, 

342. 
Momentum, 22, 50. 
Monoceros, 240. 
Mons Maenalus, 221. 

Mensae, 259. 
Month, its length according to law, 382. 
Moon, 66. 

always in the zodiac, 116. 

ancients attributed changes of weather 
to, 365. 

angular motion of, 74. 

appearance of the heavens at, 7T. 

atmosphere of, imperceptible, 78, 87. 

attracted by the Sun, 78. 

attractive power of, 67, 77. 

attracts the Earth, 117. 

can conceal the orb of the Sun, 127. 

circular depressions on, 84. 

conceals planets and stars, 138. 

cycle of, 114, 166. 

density of, 68. 

diameter of, 67. 

disc of, variegated, 69. 

discoveries on, 80. 

distance of, from the Earth, 67. 
how obtained, 174. 

form of, 68. 

full, 71. 

gibbous, 73. 

harvest, 79, 

has no clouds, 78. 

has she an atmosphere? 368. 

hunter's, 79. 

inclination of her axis, 71, 76. 
orbit, 76, 386. 

irregularities of her motions, 78, 365. 

libration in latitude, 70. 
in longitude, 70. 

lights and shadows on, 79. 

luminous spot on, 368. 

mass of, 68. 

motions of, 68. 

known to the ancients, 324. 
nodes, 387. 

mountains of, higher than ours, 83. 

new, 72. 

no clouds around it, 86. 

nodes of, 77, 125. 

no gradation of light on, 87. 

no water on its surface, 86. 

of Xeptune, 64. 

one-half of, never seen by us, 69. 

orbit of, elliptical, 74. 

inclined to the ecliptic, 126. 

parallax of, 174. 

phases of, 73. 



Moon, retained in her orbit, 77. 

revolution of, on her axis, 68, 69. 

rises later every day, 78, 119. 

same side always toward us, 69. 

seasons of, 77. 

shadows on, 82. 

shadow of, on the Earth, 13C. 

sidereal revolution of, 75. 

sometimes nearer to us than at others, 

75. 
spots on her surface, 66. 
synodic revolution of, 76. 
to find the time of southing, &c., 

Prob. XIX., 318. 
upheld in space, how, 75. 
velocitv of, in her orbit, 74. 
Moonlight, ^6 7. 

heat of, observations on, 77, 364. 
on the Moon, 363. 
Moons of Jupiter, 60. 
Saturn, 62. 
Uranus, 63, 93. 

motions of, 93. 
names of, 94, 
orbits of, 94. 
Mosotti-Lavagna, his opinion of heat of 

moonlight, 365. 
Motion, fixed stars subject to, 159. 
hourly of the Earth, 162. 
kinds of, 21. 
laws of, 21, 347. 
of Earth, 96. 

fluid bodies, 117. 
light progressive, 170. 
Moon's nodes, 387. 
tidal wave, 118. 
proper, 397. 

rapid, of some stars, 172. 
Motions, apparent, of the heavens, 99. 
heavenly bodies, 162. 
of the Moon, 68. 
irregular, 365. 
Mountains, lunar, 83. 
Miiller. Prof., microscopic observations of, 

345. 
Multiple stars, 189. 
Mural circle, 281. 

quadrant used by Ptolemy, 328. 
Musca Australis, 254. 
Musva, a Christian physician, 330. 

]^ADIR, 11. 

Xapier, Lord, invented logarithms, 337. 
Xassir Eddin, works of, 331. 
Natural day, 109, 110. 
Xeap tides, 121. 

Nebula, globular in Leo Minor, 21^. 
in Andromeda, 234. 

Aquarius, 211. 

Argo, 247. 

Auriga, 239. 

Canes Yenatici, 219, 409. 

Centaurus, 249. 

Cetus, 256. 



INDEX. 



493 



Nebula, in Cygnus, 231. 
Draco, 225. 
Fornax, 258. 
Lyra, 226. 
Orion, 242. 

Scutum Sobieski, 227. 
Triangulum, 237. 
Ursa Major, 215. 
Vulpecula, 230. 

planetary in Aquarius, 211. 

spiral in Lacerta, 232. 

split, 249. 
Nebula, 191. 

composed of stars, 194. 

divided, bow, 191. 
Nebulosity of comets, 146. 
Nebulous stars, 191. 
Negative eye-piece, 269. 
Neptune, 63. 

diameter of, 64. 

distance from tbe Sun, 64. 

first seen, bj-^ whom, 64, 

influence on Uranus explained, 362. 

moon of, 64. 

our Earth invisible from, 363. 

revolution of, round the Sun, 64. 

rotation on his axis, 64. 

seasons of, 64. 

year of, 64 
Neros, 323. 

Newfoundland, winter days in, 107. 
New Moon, 72. 
Newton, 338. 

determined the orbits of comets, 390. 

explained variation, 368. 

his discovery of gravitation, 346. 

observations made by, 158. 
Newtonian telescope, 271. 
Newton's laws, first appearance of, 347. 
New star in Cassiopeia, 235. 

Style, 381. 
Nichol, his description of a cluster, 222. 
Nights of winter longer than the days, 

106. 
Node, longitude of, 141, 142. 
Nodes, motion of Moon's, 387. 

of Mercury, 136. 
the Moon, 77, 125. 
Nonius improved the quadrant, 335. 

inventions of, 333. 
Noon, to places on the same meridian, 114. 
Noriet, measurements by, 326. 
Norma, 251. 

North pole, when in continual sunlight, 
107. 

darkness, 107. 
Northern constellations, 212, 213. 

stream, 256. 
Nubecula, 257. 

Major, 195. 

Minor, 195, 259. 
Nubeculae, 194. 
Nucleus of a comet, 140, 144, 145. 

comet of 1843, 151. 



Nucleus, planetary, 145. 
Number of Direction, 382. 

stars, 160. 
Nutation, 164, 394. 

its effect, 165. 

I Observatory at Washington, 266. 
best site for, 267. 
erection of, at Greenwich, 339. 

Paris, 338. 
of Landgrave of Hesse, 333. 
Maragha, 335. 
Tycho Brahe, 333. 
Occultation of planets, 139. 

stars, 139, 182. 
Occultations, 138. 
Octans, 252. 

Odoriferous particles material, 15. 
Ofllcina Typographia, 244. 
Olbers discovers Pallas and Vesta, 343, 
357. 
his remarks on comets, 147. 
Le Verrier's opinion of his theory, 360. 
Old Style, 380. 

Olmsted, his theory of zodiacal light, 198. 
Omar Cheyham, his reformation of the 

calendar, 331. 
Omega Centauri, 248. 
Omicron Ceti, 255. 
Ophiuehus, 223. 
Optically double stars, 188. 
Orbit, inclination of comets', 141, 142. 
Moon's, 126, 386. 
of Biela's comet, 153. 
Earth, 104. 
diameter of, how known, 176. 
elliptical, 107. 
Lexell's comet, 154. 
Orbits of asteroids, 57. 
binary stars, 187. 
comets, 140, 389. 
Jupiter's satellites, 88. 
satellites of Uranus, 94. 
Origin, supposed, of asteroids, 359. 
Orion, 240. 

Pallas discovered by Olbers, 343, 357. 
Papyrus, account of a, 323. 
Parabola, 25. 

Parallactic angle, 177, 179. 
Parallax, 137, 164, 174, 398. 

a body in the zenith has none, 176. 

annual, 177, 178. 

at its maximum, when, 174. 

eff"ect of, 175. 

horizontal, 136, 174. 

its use, 174. 

of the Moon, 174. 
stars, 174. 
Sun, 174. 
Parallel lines, 12. 
Parallelogram, 12. 
Paris observatory, erection of, 338. 
Partial eclipse, 128. 



494 



INDEX. 



Paschal cycle, 382. 

Passage, perihelion, 141, 142. 

Path of solar spots at different times, 350. 

Patonillet, Indian tables of, 321. 

Pavo, 252. 

Pearson's Practical Astronomy, 281. 

Pegasus, 233. 

square of, 233. 
Peirce, Prof., calculations by, 363. 

on the constitution of Saturn's rings, 
373. 
Pencil of light, 261. 
Pendulum applied to clocks, 335. 

for astronomical clock, 279. 
Penumbra, 124, 133. 

usually surrounds solar spots, 350. 
Perihelion, 47. 

and aphelion of asteroids, 360. 

distance, 141, 142. 

longitude of, 141, 142. 

of Earth's orbit, 108. 

passage, 141, 142. 
Period, Calippic, 114, 326. 

of comet of 1811, 149. 

1843, 151. 

1844, 149. 

sothic or canicular, 322. 
Periodic stars, 184. 
Periods of Saturn's moons, 92. 
Perioeci of a given place found, Prob. X., 

291. 
Perpendicular line, 12. 
Perseus, 237. 

Persians, astronomy of, 331. 
Petavius, a lunar cavity, 85. 
Phad, 214, 217. 
Phases of Jupiter's satellites, 89. 

Saturn's moons, 93. 

the Moon, 73. 
Phenomenon attending a total eclipse, 129. 
Philopenus, his theory of the heavenly 

bodies, 346. 
Phoenician astronomy, 324. 
Phoenix, 253. 
Photometer, 182, 261. 
Physical astronomy, 15. 
Physically double stars, 188. 
Piazzi discovers Ceres, 341, 343, 357. 
Picard, 341. 

Pilgrim, observations of, on heat of moon- 
light, 365. 
Pisces, 212. 

Australis, 253. 
Place, true, of a moving body found, 171. 
Plane mirror, reflection of light from, 264. 
Planet, how sustained in its orbit, 354. 
Planetary hours, 379. 

nebula in Aquarius, 211. 

nebulae, 191. 

magnitude of, 193. 

nuclei, 145. 

system, 155. 
Planets, 30. 

diameters of, measured, 351. 



Planets, discs magnified, 182. 

exterior, 51. 

found in the zodiac, 116. 

inferior, 35. 

interior, 34, 

names of exterior, 52. 

occultation of, 139. 

seen in the zodiac, 57. 

to find those visible after sunset, Prob. 
XIV., 315. 

ultra-zodiacal, 56. 

visible to naked eye, 159. 
Plano-concave lens, 262. 

convex lens, 262. 
Platinum wire, Wollaston's, 160. 
Plato's theory of falling bodies, 346. 
Pleiades, 201. 

a cluster, 190. 
Plumb-line, 277. 
Pointers, 214. 
Polar axis of a telescope, 270. 

circles, 11. 

on Venus, 353. 
Pole, North, in darkness, when, 107. 

in continual sunlight, when, 107. 

star, 163, 215. 
double, 217. 

4000 years ago, 225, 409. 
10,000 years hence, 226. 
place found, how, 235. 
position of, changing, 165. 
Poles, length of days at, 107. 

longest day at, 107. 

of the Earth, 45. 
heavens, 162. 
Pollux, 203. 
Portable transit, 270. 
Posidonius, observations of, 326. 
Positions, different, of the equator, 1 03. 
Positive eye-piece, 269. 
Practical Astronomy, 260. 
Praesepe, 204. 
Praxiteles, 244. 
Precession, 164, 201, 393. 

effect of, 164. 

rate of, 164. 
Prime vertical, 11. 
Principia, 339. 

doctrines of, long neglected, 347. 
Prism, 263. 
Problems on the celestial globe, 307. 

terrestrial globe, 287. 
Procyon, 242. 
Projectile, 24. 

force, 26. 
Proper motion, 164, 172, 397. 

cause of, 172. 
Psalterium Georgianum, 256. 
Ptolemaic system, 155. 
Ptolemy, 155. 

Claudius, describes the armil, 328. 

horoscope at the time of, 323. 

theory taught by, 329. 

used the mural quadrant, 328. 



INDEX. 



495 



Puppa, 244. 

Pyramids face the cardinal points, 322. 

Pythagoras, discoveries of, 325. 

system taught by, 157. 
Pytheas, his travels, 325. 
Pyxis Nautica, 246. 

Quadrant improved by Nonius, 335. 
of Tycho Brahe, 341. 
Ulugh Begh, 335. 
Quetelet, his observations of Saturn's ring, 

372. 
Quadruple stars, 190. 

Radii vectores, 48. 

Rake, 241. 

Rasmden, his invention of a ghost, 277. 

Ramsden's instruments, 342. 

Ras Algethi, 222. 

Alhague, 223. 
Rastaben, 224. 
Rate of precession, 164. 
Rational horizon, 97. 
Rawlinson, Col., memoir by, 323. 
Ray of light, 261. 

composition of, 263. 

direction of, 261. 
Rays of the Sun oblique, 105. 

perpendicular, 105. 
Reading microscopes, 283. 
Red rays the least refrangible, 264. 
Reflecting telescope, 271. 
Reflection, angle of, 264. 

of light, 264. 
Refracting telescope, 268. 
Refraction, 1 64, 166, 168, 261, 395. 

effect of, 168. 

greatest at the horizon, 168. 
Registering machine of electric clock, 280. 
Regulus, 217. 
Relative motion, 21. 
Remarkable comets, 147. 
Resultant, 23. 
Reid, 256. 

Reticulus Rhomboidalis, 259. 
Retrogradation of equinoxes, 164. 
Revolution of equinoxes, 164. 
Rhea, the fifth satellite of Saturn, 376. 
Riding level, 276. 
Rigel, 241. 
Right angle, 28, 

ascension, 13. 
Rings and moons of Saturn, 90. 

of Saturn, appearance of, 91, 
distance of, 91. 
nature of, 371. 
number of, 90. 
parallel, 91. 
revolution of, 90, 91. 
thickness of, 91. 
Rittenhouso first employed the collimator, 

278. 
Robur Caroli, 246. 
Roemer, discoveries of, 89, 170, 338. 



Roman day, 379. 

indication, 382. 
Rosse, Earl of, his telescope, 273. 
Rudolphine tables, 336. 
Rule for finding leap year, 113. 
RUmker, Madame, discovered a comet, 391. 

Sagitta, 229. 

Sagittarius, 209. 

Sails, 244. 

Sandwich Islands, observations of transit 

at, 138. 
Saros, 323. 
Satellites, 30, 65. 

discovery of Jupiter's, 88. 
eclipses of Jupiter's, 369. 
form of their orbits, 65. 
Jupiter's, exhibit phases, 89. 
number of, 66. 

Jupiter's, 88. 
of Saturn, 375. 
orbits of Jupiter's, 88. 
rapid motion of Jupiter's, 90. 
Saturn, 61. 

appearance of, 61. 

rings, 91. 
dark ring of, 374. 

transparent, 375. 
diameter of, 61. 
distance of, from the Sun, 61. 
rings from the planet, 91. 
inclined to his orbit, 62. 
inner dark ring of, 90. 
known to the ancients, 374. 
measurement of rings, 373. 
number of rings, 90. 
outer ring of, 90. 
period of rotation of, 61. 
revolution of rings, 91. 
rings of, 61. 

described, 90. 
parallel, 91. 
rotation of, 90. 
supposed to be fluid, 373. 
satellites of, 62, 92, 375. 
eclipses of, 92. 
periods of, 92. 
phases of, 93. 
seasons of, 62. 
teloscopic views of, 90. 
thickness of rings, 91. 
use of rings, 91. 
year of, 61. 
Sceptrum Brandenburgium, 243, 257. 
Scheat, 211, 233. 
Scheiner on the solar spots, 337. 
Schlegel, 321. 
Schiibler, meteorological observations of, 

365. 
Scintillation of stars, 158. 
Scoresby, his account of refraction, 396. 
Scorpio, 207. 
Scutum Sobieski, 227. 
Seasons, 102. 



496 



INDEX. 



Seasons, cause of, 41, 55, 103. 
changes of, 103. 

how designated by Hesiod, 324. 
of Mars, 55. 
Moon, 77. 
Saturn, 62. 
Venus, 351. 
Seginus, 220. 

Seneca, his account of a comet, 143. 
Sensible horizon, 97. 
Serpens, 223. 
Sessos, 323. 
Seven Stars, 202. 
Sextans, 247. 
Shadow defined, 124. 
direction of, 124. 
form of Earth's, 131. 
length of Earth's, 131. 
Shadows, conical, 124. 
decrease of lunar, 84. 
length of, 80. 

none under a vertical Sun, 81. 
on the Moon, 82. 
Shakerly observes the transit of Mercury, 

387. 
Sharatan, 201. 
Sh^dir a variable star, 235. 
Shemeli, 256. 
Shooting stars, 195. 
Skort period, comets of, 142. 
Sidereal astronomy, 158. 
day, 44, 109. 

revolution of the Moon, 75. 
time, 109. 
year, 112, 201. 
Signs of the zodiac, 116. 
Sine, 14. 
Sirius, 243. 

appearance through the telescope, 183. 
color of, changed, 184. 
distance from us, 183. 
light of, 161, 243. 

compared to the Sun, 183. 
size estimated, 183. 
Sky, 161. 

Smyth, Admiral, opinion of Hindu science, 
321. 
observations of the Moon, 368. 
Snell discovered the law of sines, 395, 
Solar and lunar tidal waves, 385. 

sidereal day, difference of, 110. 
day, 110. 

common reckoning of time, 110. 
length of, 109, 110. 
eclipse, 125. 

cause of, 127. 

over what extent visible, 129. 
ecliptic limit, 131. 
spectrum, 264. 
spots, 349. 

seen by Scheiner, 337. 
size of, 33. 
system, 29, 155. 
time, 109. 



Solar time, common clocks show mean, 110. 
Solarium, 259, 

Solid bodies, movements of, 117. 
Solstice, summer, 108. 

winter, 108. 
Solstices, 13, 115, 
Solstitial armil, 328. 

colure, 115. 
Somerville, Mrs., account of a papyrus, 323. 
Sothic period, 322. 
South pole of the heavens, 252. 
Southern constellations, 240. 

Stream, 256. 
Specific gravity, 21. 
Specula, 264. 
Speculum, 271. 
Sphere, 11. 
Spheroid, 11. 
Spica, 205. 

Spider -line micrometer, 274, 283. 
Spirit level, 276, 
Split nebula, 249. 
Spots, solar, 349. 
Spring and neap tides, 385. 
compared, 122. 
tides, 131. 
Square of Pegasus, 233. 
Star, its altitude, &c. given, to find the 
azimuth, Prob. VIIL, 310. 
its rising, setting, &c. found, Prob. 

XL, 312, 
new, A,D. 389, 186. 

1572, 186. 
number 61 Cygni, 172. 
occultation of, 139. 

to find its altitude, &c., Prob. IX,, 311. 
in a given latitude, Prob,, VII,, 
310, 
what day it passes the meridian, 
Prob. v., 309. 
hour it comes to the meridian, 
Prob. IV., 308. 
when it culminates, Prob. XV., 316. 
Stars, apparent diameters of, 159. 
appearance of, 182, 184, 
brilliancy of, measured, 182. 
catalogues of, 161, 
circumpolar, 213, 
classification of, 160, 
complementary colors of, 188, 
compound, 187. 
designation of, 200. 
discs of, not measurable, 139, 182. 
double, 187. 

elliptical motion of some, 187. 
exhibit different colors, 184, 
fixed, 158, 159, 

useful to us, 159. 
immensely distant from us, 159, 174. 
light of, measured, 161. 

compared with the Sun, 183. 
magnitude of, 160, 182. 
maps of, 161. 
number of, 160. 



INDEX. 



497 



Stars, optically double, 188. 
perfect cbronometers, 159. 
physically double, 188. 
remarkable periodic, 184. 
sMne by their native light, 158. 
shooting, 195. 
some invisible to the naked eye, 180. 

revolve round each other, 187. 
subject to slight motion, 159. 
telescopic, 161. 
to find those which never rise or set at 

a given place, Prob. XIIL, 314. 
variation of light of, 185. 
visible in daytime, 100. 
Stellar light unpolarized, 158. 
nebulae, 191, 193. 
universe, 159. 
Stern, 244. 

St. Malo, tides at, 123. 
Strabo speaks of the observatory at Cni- 

dus, 325. 
Strata, atmospheric, around the Sun, 349. 
Stream, Northern and Southern, 256. 
Struve, his estimates of number of stars, 
180. 
observations of Halley's comet, 391. 
Style, New and Old, 380, 381. 
Substance of comets, 144. 
Summer, and winter, on the Moon, 364, 

cause of long days in, 105, 106. 
Sun, 30. 

altitude and azimuth of, on a certain 
day, Prob. XVIIL, 317. 

of, for any hour, found, Prob, 
XVII., 296. 
always in the zodiac, 116. 
appearance of, through the telescope, 

350. 
appears behind his true place, 172. 
atmosphere of, 349. 
attraction of, 48. 
attractive power of, 385. 
attracts the Moon, 117. 

waters of the sea, 120. 
bx'eadth of his disc, 47. 
cycle of, 114. 

diameter of, greatest in winter, 377. 
distance of, how discovered, 137. 
in winter and summer, 377. 
'* drawing water" explained, 369. 
form of, 34. 

in the foci of a comet's orbit, 141. 
light of, compared with that of stars, 

183. 
light of, compared to Sirius, 183. 
magnitude of, 34. 

meridian altitude of, found, Prob. 
XVI., 295. 

given, to find latitude, Prob. XIX., 
297. 
motion of, in space, 173. 
nearer to us in winter, 107. 
never seen in his true place, 172. 
object of worship in early times, 320. 



Sun obscures the light of stars, 100. 
orb of, concealed by Moon, 127. 
parallax of, 174. 
place of, found, Prob. XIIL, 293. 

to rectify the globe for, Prob. 
XIV., 294. 
remarkable obscurations of, 404. 
revolution of, on its axis, 34. 
rising and setting of, found, Prob. 

XV., 294. 
situated in the Milky "Way, 401. 
situation of, 181. 
those places where he has the same 

altitude, Prob. XVIII., 296. 
to find his right ascension on a given 
day, Prob. XVI., 316. 

oblique ascension on a given day, 

Prob. XVIL, 317. 
what two days he is vertical at 
the same place in the torrid 
zone, Prob. XXL, 299. 
the latitude in the frigid zone, 
where he does not set for a 
given number of days, Prob. 
XXVIL, 304. 
when he is due east or west, the 
latitude and day being given, 
Prob. XX., 298. 
where he is rising, <fec., the day and 
hour being given, Prob. XXIV., 
30L 
vertical, the day and hour 
being given, Prob. XXIIL, 
300. 
in the frigid zone, he begins to 
shine without setting on a given 
day, Prob. XXVL, 303. 
vertical within the tropics, 106. 
visible when below thd horizon, 368. 
volume of, 34. 
Sunrise and sunset, cause of, 96. 
Syene, well ©f, 326. 
Synodic revolution of the Moon, 76. 
System, Copernican, 157. 
Egyptian, 156. 
planetary, 155. 
Ptolemaic, 155. 
solar, 155. 
Tychonic, 156, 333. 
Systems, binary, 187. 
of astronomy, 155. 

TABiiES, Alphonsine, 381, 332. 

astronomical, of India, 321. 

Rudolphine, 336. 
Tail of comet of 1843, 151. 
Tails of comets, 140, 146, 391. 
Tangent, 14. 
Tarandus, 238. 
Tarazed, 229. 

Tartars, astronomy of, 331. 
Taurus, 201. 

Poniatowski, 227. 
Teachers, note to, 198. 



32 



498 



INDEX. 



Tegmine, 204. 

Telescope, achromatic, 342. 

aids our sight, 80. 

Earl of Rosse's, 273. 

eflfect of, on the stars, 182. 

equatorial, 270. 

magnifies discs of planets, 182. 

reflecting, 271. 

reveals numbers of stars, 180. 

Sir W. Herschel's, 273. 
Telescopes, 268. 

in observations, 268. 
Telescopic comets, 146, 390. 

power directed to the Moon, 80. 

stars, ]61. 

views of Saturn, 90. 
Telescopium, 251. 

Ilerschelii, 239. 
Temporary stars, 184, 185. 
Tentyris, 323. 
Thales erected a gnomon at Sparta, 327. 

taught the Greeks astronomy, 324. 
Thebit, a lunar cavity, described, 85. 
Theon mentions the use of water-level, 328. 
Thoth, Egyptian name for Sirius, 322. 
Three rings, 241. 
Thuban, 225. 
Tidal wave, 117. 

height of, 120. 

higher under the Moon, 123. 

highest rise of, 122. 

motion of, 118. 
Tides, 117. 

cause of, 384. 

caused by Moon, 67. 

height increased, when, 385. 

higher in channels, 123. 

in the river Amazon, 386. 

lagging of, 117. 

neap, 121. 

occur later every day, 119. 

principal sources of, 123. 

spring, 121. 

and neap, 385. 

two in twenty-four hours, 119. 

use of, 123. 
Time, apparent, defined, 111. 

ball. 111. 

decimal divisions used in China, 322. 

defined, 108. 

ditierence of, between two places, 
Prob. v., 289. 

different kinds of, 109. 

divisions of, 109. 

equation of, 112. 

mean, 111. 

measure of, 108. 

sidereal, 109. 

solar, 109. 
Timocharis and Aristillus, observations of, 

326. 
Titan, the sixth satellite of Saturn, 375. 
Toaldo on the heat of moonlight, 365. 
Toscanelli erected a gnomon at Florence, 32 7. 



Total eclipse, 128. 

duration of, 129. 

occurs, when, 130. 
Toucana, 253. 
Toucani, 47, 254. 
Transit, 41, 134. . 

appearance produced by, 135. 

circle, 284. 

clock, 279. 

instrument, 270. 

of Mercury, 136. 
Venus, 136. 

first recorded, 137. 
Transits, can occur, when, 134. 

prove what, 135. 

of Mercury, 136, 387. 
occur, when, 136. 
Transverse axis, 13. 
Triangle, 12. 
Triangulum, 236. 

Australis, 252. 

nebula in, 237. 
Tropical year, 112. 
Tropics, 11. 

place of, on Venus, 353. 

Sun vertical within, 106. 
Twilight, 169. 

on the Moon, 369. 
Twinkling of the stars, 158. 
Tycho, a lunar cavity, described, 84, 85. 

Brahe, 156, 333. 

observatory of, destroyed, 247. 
Tychonic system, 156, 333. 

ULTRA-zodiacal planets, 56. 
Ulugh Begh, works of, 331. 
Umbra, 133. 

Undulatory motion of light, 260. 
Universe, stellar, 159. 
Unuk-al-Hay, 223. 
Uranibourg, 333. 
Uranus, 63. 

also called Herschel, 63. 

diameter of, 63. 

discoverer of, 63. 

discovery of, 340, 361. 

distance from the Sun, 63. 

first observation of, 361. 

moons ,of, 63, 93, 94. 

orbits of moons, 94. 

period of, 63. 

revolution on his axis, 63. 
Ursa Major, 213. 

Minor, 215. 

four positions of, 216. 

Variable stars, 184. 

Variation explained, 185, 367, 368. 

of light of stars, 185. 
Varro, his predictions of the weather, 365. 
Velis, 244. 
Velocities of falling bodies, 345. 

planets, 41. 
Velocity of a body, 26. 



INDEX. 



499 



Velocity of a ray of light, 89. 

bodies of unequal weight, 18. 

falling bodies, 18. 

the Earth, 50, 112. 

greatest near the Sun, 108. 

61 Cygni, 172. 
Venus, 37. 

a full orb, 40. 
morning and evening star, 41. 

dianaeter of, 39. 

distance of, from the Earth, 39. 
Sun, 39. 

first transit of, seen by man, 337. 

greatest elongation of, 38. 

has no moon, 42. 

inclination of axis of, 41. 

length of year, 40. 

never on the meridian at midnight, 
52. 

period of, 40. 

seasons of, 351. 

synodic revolution of, 40. 

transits of, 136. 

first recorded, 137. 

velocity of, 41. 
Vernal equinox, 164. 
Vernier, 275. 
• Vertical circles, 11. 

collimator, 278. 
Vesta discovered by Olbers, 843, 357. 
Via Lac tea, 180. 
Vindemiatrix, 205. 

Virgil, his predictions of the weather, 365. 
Virgo, 205. 

nebula in, 206. 
Vortices of Descartes, 334. 
Vulpecula et Anser, 229. 

Wagon, 213. 

Walker, Sears C, his investigations, 363. 

Wallingford, his clock, 335. 

Walther first used clocks in observations, 
335. 

Wasat, 203. 

Washington observatory, 266. 

Wave, height of tidal, 120. 
motion of tidal, 118. 
tidal, 117. 

Week a period of time used by the an- 
cients, 379. 

Wega, 225. 

pole star, when, 165. 



Weight, 49. 

Weights and measures, standard of, 353. 

Whiston, his observations of Saturn's 

ring, 372. 
White Ox, 195. 
Wilson, his theory of motion of the Sun, 

397. 
Winchmann, his observations, 153. 
Winds influence tides, 123. 
Winlock, Prof., his observations on the 

Moon, 369. 
Winter, days short in, 105. 

nights longer than days, 106. 
solstice, 108. 
Wire, micrometer, 274. 
Wollaston, experiments by, 161, 396. 
platinum wire made by, 160. 

Year, 102. 

astronomical, 382. 

begins with the commencement of a 

day, 113. 
civil, 381. 
defined, 112. 
Egyptian, 322. 
Gregorian, 381. 
Julian, 327. 
length determined by the ancients, 

380. 
length of a planet's, 53. 

zodiacal, 198. 
on Uranus, 63. 
sidereal, 112. 
tropical, 112. 

Zach, Baron de, his computations, 357. 
Zaurak, 257. 
Zavijava, 206. 
Zenith, 11, 98. 

a body in, has no parallax, 176. 
Zodiac, 116, 201. 

how marked on the globe, 116. 

Indian, 321. 

meaning of, ] 16. 

Moon always in, 116. 

of Tentyris, 323. 

planets seen in, 57. 

Sun always in, 116. 
Zodiacal constellations, 201, 

light, 198, 405. 
Zuben-el-Gamabi, 207. 
Zubeneschamali, 207. 



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